Magneto-optical recording medium and reproducing method for information recorded on the medium

ABSTRACT

A magneto-optical recording medium capable of performing magnetically induced super resolution reproduction by perfectly masking a mark adjacent to a mark to be reproduced. The magneto-optical recording medium includes a transparent substrate, a magnetic reproducing layer laminated on the transparent substrate, a magnetic control layer laminated on the reproducing layer, and a magnetic recording layer laminated on the control layer. The reproducing layer has an easy axis of magnetization in a plane at room temperature. Each of the control layer and the recording layer has an easy axis of magnetization perpendicular to the substrate. When a given reproducing power is applied to the medium, there are formed in a laser spot on the medium a low-temperature area where the direction of magnetization of the reproducing layer is made identical with an in-plane direction, an intermediate-temperature area where the magnetization of the recording layer is transferred through the control layer to the reproducing layer by exchange bond, and a high-temperature area whose temperature is higher than the Curie temperature of the control layer. In the low-temperature area, an in-plane mask is formed. In the high-temperature area, an up spin mask is formed. Further, an opening allowing a magneto-optical signal to be read is formed between the two masks.

This is a continuation of application Ser. No. 08/368,611, filed Jan. 4,1995 now U.S. Pat. No. 5,723,277.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-density magneto-opticalrecording medium and a reproducing method for information recorded onthe medium.

2. Description of the Related Art

A magneto-optical disk is known as a high-density recording medium, andan increase in information quantity gives rise to a desire for higherdensities of the medium. While the higher densities may be realized byreducing the space of recorded marks, the recording and reproducing ofthe marks are limited by the size of a light beam (beam spot) on themedium. When the presence of only one recorded mark in the beam spot isset, an output waveform corresponding to "1" or "0" may be observed as areproduction signal according to whether the recorded mark is present orabsent in the beam spot.

However, when the presence of plural recorded marks in the beam spot isset by reducing the space of the recorded marks, no change inreproduction output occurs regardless of movement of the beam spot onthe medium. Accordingly, the output waveform becomes linear and thepresence or absence of the recorded mark in the beam spot cannot beidentified. To reproduce such small recorded marks having a periodsmaller than the size of the beam spot, it is necessary to reduce thebeam spot to a small size. However, since the size of the beam spot islimited by the wavelength λ of a light source and the numerical apertureNA of an objective lens, the beam spot cannot be sufficiently reduced toa small size.

A reproducing method has recently been proposed magnetically inducedsuper resolution such that a recorded mark smaller in size than the beamspot can be reproduced by the use of an existing optical system.According to this method, the resolution of reproduction is improved bymasking other marks during reproduction of one mark in the beam spot.Accordingly, a super resolution disk medium is required to have at leasta mask layer or a reproducing layer for masking other marks so that onlyone mark may be reproduced during signal reproduction, in addition to arecording layer for recording marks.

A magneto-optical recording medium using a perpendicular magnetizationfilm as the reproducing layer is proposed in Japanese Patent Laid-openNo. 3-88156. In the prior art described in this publication, however, aninitializing magnetic field of about several kOe is required toinitialize the reproducing layer. Accordingly, a compact recordingapparatus cannot be made. On the other hand, a magneto-optical recordingmedium using a magnetic film as the recording layer is proposed inJapanese Patent Laid-open No. 5-81717. This magnetic film has an easyaxis of magnetization in a plane at room temperature and has an easyaxis of magnetization perpendicular to a film surface at a giventemperature or higher.

The principle of reproduction in this prior art will now be described inbrief with reference to FIGS. 28A, 28B, and 28C. As shown in FIG. 28C, amagneto-optical disk 2 is formed by laminating a magnetic reproducinglayer 6 and a magnetic recording layer 8 on a transparent substrate 4.The magnetic reproducing layer 6 has an easy axis of magnetization in aplane at room temperature. However, when the medium is heated to a giventemperature or higher by applying a reproducing power, the easy axis ofmagnetization is changed to a perpendicular direction. The magneticrecording layer 8 is a perpendicular magnetization film. Referencenumeral 10 denotes a light beam.

The intensity distribution of the light beam is a Gaussian distributionas shown in FIG. 28A. Accordingly, when the disk is at rest, thetemperature distribution on the disk is also a similar distribution suchthat the central portion is higher in temperature than the peripheralportion. In actuality, however, the disk 2 is rotated in the directionof arrow R shown in FIG. 28C during reproduction. Accordingly, thetemperature distribution on the disk in rotation becomes a distributionas shown in FIG. 28B so that a high-temperature area in the beam spot isshifted to the forward direction of rotation of the disk. Owing to sucha temperature distribution during reproduction, the easy axis ofmagnetization of the magnetic reproducing layer 6 becomes an in-planedirection in a low-temperature area in the beam spot. Therefore, theKerr rotation angle of reflected light becomes almost zero in thelow-temperature area. In the high-temperature area, the easy axis ofmagnetization of the magnetic reproducing layer 6 is changed from anin-plane direction to an perpendicular direction.

The perpendicular magnetization of the magnetic reproducing layer 6 atthis time is bonded to the magnetization of the magnetic recording layer8 by an exchange force, and the direction of magnetization of thereproducing layer 6 is made identical with the direction ofmagnetization of the recording layer 8, thereby allowing themagnetization recorded in the recording layer 8 to be transferred to thereproducing layer 6. The area size of such transfer can be changed by areproducing layer beam power or the rotation of the disk. In thismanner, the size of the masking reproducing layer is controlled so as toallow the reproduction of only one recorded mark, thereby obtaining thesame effect as that in the case of substantially reducing the area ofthe beam spot to improve the resolution and realize high-densityrecording and reproduction.

As mentioned above, the intensity distribution of the laser beam 10directed onto the disk 2 is a Gaussian distribution, and the disk 2 isrotated in the direction of arrow R. As a result, a low-temperature areaand a high-temperature area are formed on the reproducing layer 6. Thehigh-temperature area is shifted to the downstream side or the trailingside of the beam spot formed on the disk. In this manner, thehigh-temperature area where information is reproduced is shifted to thedownstream side in the beam spot, so that the intensity of the laserbeam is relatively reduced. Thus, the prior art described in JapanesePatent Laid-open No. 5-81717 cannot obtain a large magneto-opticalsignal output. Further, as an optical mask is formed at only theupstream side in the beam spot, an opening for reproducing informationcannot be reduced in size.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide ahigh-density magneto-optical recording medium which can accuratelyrecord a mark smaller than that in the prior art and can reproduce itwith a greater magneto-optical signal.

It is another object of the present invention to provide an informationreproducing method which can accurately reproduce information recordedwith a high density on the magneto-optical recording medium.

According to a first aspect of the present invention, there is provideda magneto-optical recording medium comprising a transparent substrate; afirst magnetic layer laminated on said transparent substrate and havingan easy axis of magnetization in a plane at room temperature; a secondmagnetic layer laminated on said first magnetic layer and having an easyaxis of magnetization perpendicular to said substrate; and a thirdmagnetic layer laminated on said second magnetic layer and having aneasy axis of magnetization perpendicular to said substrate; wherein aCurie temperature Tc1 of said first magnetic layer, a Curie temperatureTc2 of said second magnetic layer, and a Curie temperature Tc3 of saidthird magnetic layer are related to satisfy Tc1>Tc2 and Tc3>Tc2.

The second magnetic layer having the easy axis of magnetizationperpendicular to the substrate may be replaced by a second magneticlayer having an easy axis of magnetization in a plane at roomtemperature. As a modification of the first aspect, a nonmagneticintermediate layer may be interposed between the first magnetic layerand the second magnetic layer or between the second magnetic layer andthe third magnetic layer. Preferably, each magnetic layer is formed froma rare earth-transition metal amorphous alloy film. Other alloy filmsand magnetic oxide films may be adopted as each magnetic layer.

According to a second aspect of the present invention, there is provideda magneto-optical recording medium comprising a transparent substrate; afirst magnetic layer laminated on said transparent substrate and havingan easy axis of magnetization perpendicular to said substrate; a secondmagnetic layer laminated on said first magnetic layer and having an easyaxis of magnetization in a plane at room temperature; a third magneticlayer laminated on said second magnetic layer and having an easy axis ofmagnetization perpendicular to said substrate; and a fourth magneticlayer laminated on said third magnetic layer and having an easy axis ofmagnetization perpendicular to said substrate; wherein a Curietemperature Tc1 of said first magnetic layer, a Curie temperature Tc2 ofsaid second magnetic layer, a Curie temperature Tc3 of said thirdmagnetic layer, and a Curie temperature Tc4 of said fourth magneticlayer are related to satisfy Tc1>Tc3, Tc2>Tc3, and Tc4>Tc3; and acoercive force Hc1 of said first magnetic layer at room temperature, acoercive force Hc2 of said second magnetic layer at room temperature, acoercive force Hc3 of said third magnetic layer at room temperature, anda coercive force Hc4 of said fourth magnetic layer at room temperatureare related to satisfy Hc4>Hc3 and Hc4>Hc1.

According to a third aspect of the present invention, there is provideda magneto-optical recording medium comprising a transparent substrate; afirst magnetic layer laminated on said transparent substrate and havingan easy axis of magnetization perpendicular to said substrate; a secondmagnetic layer laminated on said first magnetic layer and having an easyaxis of magnetization in a plane at room temperature; and a thirdmagnetic layer laminated on said second magnetic layer and having aneasy axis of magnetization perpendicular to said substrate; wherein aCurie temperature Tc1 of said first magnetic layer, a Curie temperatureTc2 of said second magnetic layer, and a Curie temperature Tc3 of saidthird magnetic layer are related to satisfy Tc1>Tc2 and Tc3>Tc2; and acoercive force Hc1 of said first magnetic layer at room temperature anda coercive force Hc3 of said third magnetic layer at room temperatureare related to satisfy Hc3>Hc1.

According to another aspect of the present invention, there is provideda reproducing method for information recorded on a magneto-opticalrecording medium comprising a transparent substrate; a first magneticlayer laminated on said transparent substrate and having an easy axis ofmagnetization in a plane at room temperature; a second magnetic layerlaminated on said first magnetic layer and having an easy axis ofmagnetization perpendicular to said substrate; and a third magneticlayer laminated on said second magnetic layer and having an easy axis ofmagnetization perpendicular to said substrate; wherein a Curietemperature Tc1 of said first magnetic layer, a Curie temperature Tc2 ofsaid second magnetic layer, and a Curie temperature Tc3 of said thirdmagnetic layer are related to satisfy Tc1>Tc2 and Tc3>Tc2; saidreproducing method comprising the steps of directing a laser beam ontosaid recording medium as applying a bias magnetic field to heat saidrecording medium to temperatures lower than the Curie temperature ofsaid third magnetic layer: and forming a temperature distribution in abeam spot, said temperature distribution comprising a first area wherethe direction of magnetization of said first magnetic layer is anin-plane direction, a second area where magnetization of said thirdmagnetic layer is transferred to said second magnetic layer and saidfirst magnetic layer by exchange bond, and a third area where thetemperature of said second magnetic layer becomes higher than its Curietemperature.

According to still another aspect of the present invention, there isprovided a reproducing method for information recorded on amagneto-optical recording medium comprising a transparent substrate; afirst magnetic layer laminated on said transparent substrate and havingan easy axis of magnetization perpendicular to said substrate; a secondmagnetic layer laminated on said first magnetic layer and having an easyaxis of magnetization in a plane at room temperature; a third magneticlayer laminated on said second magnetic layer and having an easy axis ofmagnetization perpendicular to said substrate; and a fourth magneticlayer laminated on said third magnetic layer and having an easy axis ofmagnetization perpendicular to said substrate; wherein a Curietemperature Tc1 of said first magnetic layer, a Curie temperature Tc2 ofsaid second magnetic layer, a Curie temperature Tc3 of said thirdmagnetic layer, and a Curie temperature Tc4 of said fourth magneticlayer are related to satisfy Tc1>Tc3, Tc2>Tc3, and Tc4>Tc3; and acoercive force Hc1 of said first magnetic layer at room temperature, acoercive force Hc2 of said second magnetic layer at room temperature, acoercive force Hc3 of said third magnetic layer at room temperature, anda coercive force Hc4 of said fourth magnetic layer at room temperatureare related to satisfy Hc4>Hc3 and Hc4>Hc1; said reproducing methodcomprising the steps of directing a laser beam onto said recordingmedium as applying a bias magnetic field to heat said recording mediumto temperatures lower than the Curie temperature of said fourth magneticlayer; and forming a temperature distribution in a beam spot, saidtemperature distribution comprising a first area where the direction ofmagnetization of said second magnetic layer is an in-plane direction, asecond area where magnetization of said fourth magnetic layer istransferred to said third magnetic layer, said second magnetic layer,and said first magnetic layer by exchange bond, and a third area wherethe temperature of said third magnetic layer becomes higher than itsCurie temperature.

According to a further aspect of the present invention, there isprovided a reproducing method for information recorded on amagneto-optical recording medium comprising a transparent substrate; afirst magnetic layer laminated on said transparent substrate and havingan easy axis of magnetization perpendicular to said substrate; a secondmagnetic layer laminated on said first magnetic layer and having an easyaxis of magnetization in a plane at room temperature; and a thirdmagnetic layer laminated on said second magnetic layer and having aneasy axis of magnetization perpendicular to said substrate; wherein aCurie temperature Tc1 of said first magnetic layer, a Curie temperatureTc2 of said second magnetic layer, and a Curie temperature Tc3 of saidthird magnetic layer are related to satisfy Tc1>Tc2 and Tc3>Tc2; and acoercive force Hc1 of said first magnetic layer at room temperature anda coercive force Hc3 of said third magnetic layer at room temperatureare related to satisfy Hc3>Hc1; said reproducing method comprising thesteps of directing a laser beam onto said recording medium as applying abias magnetic field to heat said recording medium to temperatures lowerthan the Curie temperature of said third magnetic layer; and forming atemperature distribution in a beam spot, said temperature distributioncomprising a first area where the direction of magnetization of saidfirst magnetic layer is identical with the direction of said biasmagnetic field, a second area where magnetization of said third magneticlayer is transferred to said second magnetic layer and said firstmagnetic layer by exchange bond, and a third area where the temperatureof said second magnetic layer becomes higher than its Curie temperatureand the direction of said first magnetic layer is identical with thedirection of said bias magnetic field.

The information reproducing method of the present invention ischaracterized in that in reproducing information recorded on themagneto-optical recording medium having the first, second, and thirdmagnetic layers, a mark smaller than that in the prior art is reproducedaccurately by utilizing a temperature gradient generated in the beamspot formed on the recording medium. In a low-temperature area in thebeam spot, the direction of magnetization of the first magnetic layer ismade identical with the in-plane direction to form an in-plane mask. Ina high-temperature area in the beam spot whose temperature is higherthan the Curie temperature of the second magnetic layer, themagnetization of the second magnetic layer disappears and the directionof magnetization of the first magnetic layer is made identical with thedirection of the bias magnetic field to form an up spin mask or a downspin mask.

In an intermediate-temperature area (opening) in the beam spot, themagnetization of the third magnetic layer is transferred through thesecond magnetic layer to the first magnetic layer, thereby allowing theinformation recorded in the third magnetic layer to be reproduced. Thatis, in differentially detecting a magneto-optical output, thelow-temperature area and the high-temperature area in the beam spotfunction as masks to inhibit reading of a magneto-optical signal inthese areas, while a magneto-optical signal in only theintermediate-temperature area is allowed to be read. Accordingly, a markhaving a size less than the diffraction limit of a laser wavelength canbe read accurately.

Also in reproducing information recorded on the magneto-opticalrecording medium having the four magnetic layers, the principle ofreproduction is similar to that in the case of reproducing informationrecorded on the magneto-optical recording medium having the threemagnetic layers mentioned above. In this case, the first magnetic layerlaminated on the transparent substrate is a perpendicular magnetizationfilm, and the second magnetic layer for reading a signal has arelatively wide temperature area where in-plane magnetization is changedto perpendicular magnetization, resulting in reduction in C/N of areproduction output. However, the provision of the first magnetic layerthat is a perpendicular magnetization film can improve the C/N of areproduction output.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a magneto-optical recordingmedium according to a first preferred embodiment of the presentinvention;

FIG. 2 is a vertical sectional view of a magneto-optical recordingmedium according to a second preferred embodiment of the presentinvention;

FIG. 3 is a vertical sectional view of a magneto-optical recordingmedium according to a third preferred embodiment of the presentinvention;

FIG. 4 is a schematic illustration of a magneto-optical disk drive unit;

FIG. 5 is a vertical sectional view illustrating data erasing;

FIG. 6 is a vertical sectional view illustrating data writing;

FIG. 7A is a plan view illustrating a reproducing condition when thetemperature in a beam spot is lower than Tcopy:

FIG. 7B is a vertical sectional view of the recording medium in thecondition of FIG. 7A;

FIG. 8A is a plan view illustrating a reproducing condition when thetemperature in the beam spot is higher than Tcopy and lower than Tc2:

FIG. 8B is a vertical sectional view of the recording medium in thecondition of FIG. 8A;

FIG. 9A is a plan view illustrating a data reproducing method accordingto the first preferred embodiment;

FIG. 9B is a vertical sectional view of the recording medium in thecondition of FIG. 9A;

FIG. 10 is a graph showing the mark length dependency of C/N in thereproducing method according to the first preferred embodiment incomparison with the prior art;

FIG. 11 is a graph showing the reproducing power dependency of C/N;

FIG. 12A is a plan view illustrating a data reproducing method when abias magnetic field is applied downward;

FIG. 12B is a vertical sectional view of the recording medium in thecondition of FIG. 12A;

FIG. 13 is a vertical sectional view of a magneto-optical recordingmedium according to a fourth preferred embodiment of the presentinvention;

FIG. 14 is a vertical sectional view illustrating data erasing;

FIG. 15 is a vertical sectional view illustrating data writing;

FIG. 16A is a plan view illustrating a single-masked reproducing method;

FIG. 16B is a vertical sectional view of the recording medium in thecondition of FIG. 16A;

FIG. 17A is a plan view illustrating a data reproducing method accordingto the fourth preferred embodiment;

FIG. 17B is a vertical sectional view of the recording medium in thecondition of FIG. 17A;

FIG. 18 is a vertical sectional view of a magneto-optical recordingmedium according to a fifth preferred embodiment of the presentinvention;

FIG. 19 is a vertical sectional view illustrating data erasing;

FIG. 20 is a vertical sectional view illustrating data writing;

FIG. 21A is a plan view illustrating a data reproducing method accordingto the fifth preferred embodiment when a bias magnetic field is appliedupward;

FIG. 21B is a vertical sectional view of the recording medium in thecondition of FIG. 21A;

FIG. 22A is a plan view illustrating a data reproducing method accordingto the fifth preferred embodiment when a bias magnetic field is applieddownward;

FIG. 22B is a vertical sectional view of the recording medium in thecondition of FIG. 22A;

FIG. 23 is a vertical sectional view of a magneto-optical recordingmedium according to a sixth preferred embodiment of the presentinvention;

FIG. 24 is a vertical sectional view illustrating data erasing;

FIG. 25 is a vertical sectional view illustrating data writing;

FIG. 26A is a plan view illustrating a single-masked reproducing method;

FIG. 26B is a vertical sectional view of the recording medium in thecondition of FIG. 26A;

FIG. 27A is a plan view illustrating a data reproducing method accordingto the sixth preferred embodiment;

FIG. 27B is a vertical sectional view of the recording medium in thecondition of FIG. 27A; and

FIGS. 28A, 28B, and 28C are views illustrating the principle ofreproduction in the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The structure of a magneto-optical recording medium 12 according to afirst preferred embodiment of the present invention will be describedwith reference to FIG. 1. The magneto-optical recording medium 12 isusually in the form of disk. A dielectric layer 16 formed of SiN or thelike by sputtering, for example, is laminated on a transparent substrate14 formed of glass or the like. The dielectric layer 16 preventsoxidation and corrosion of a magnetic layer laminated thereon. Resinssuch as polycarbonate, polymethyl methacrylate, and amorphous olefin maybe adopted as the transparent substrate 14. Metal nitrides such as AlN,metal oxides such as SiO₂ and Al₂ O₃, and metal sulfides such as ZnS maybe adopted as the dielectric layer 16.

A magnetic reproducing layer 18 formed from a rare earth-transitionmetal amorphous alloy film such as GdFeCo is laminated on the dielectriclayer 16. The magnetic reproducing layer 18 has an easy axis ofmagnetization in a plane at room temperature. Preferably, the easy axisof magnetization of the reproducing layer 18 is changed from thein-plane direction to the perpendicular direction at temperatures higherthan a given temperature to which the layer 18 is heated by areproducing beam power. A magnetic control layer 20 formed from a rareearth-transition metal amorphous alloy film such as TbFe is laminated onthe magnetic reproducing layer 18. The magnetic control layer 20 has aneasy axis of magnetization perpendicular to the substrate 14.

A magnetic recording layer 22 formed from a rare earth-transition metalamorphous alloy film such as TbFeCo is laminated on the magnetic controllayer 20. The magnetic recording layer 22 has an easy axis ofmagnetization perpendicular to the substrate 14. A Curie temperature Tc1of the reproducing layer 18, a Curie temperature Tc2 of the controllayer 20, and a Curie temperature Tc3 of the recording layer 22 arerelated to satisfy Tc1>Tc2 and Tc3>Tc2.

A protective film 24 is laminated on the magnetic recording layer 22 tocomplete the magneto-optical recording medium 12. The protective film 24prevents entry of water, oxygen, or other substances such as halogenfrom the air to protect the magnetic recording layer 22. Metal nitridessuch as SiN and AlN, metal oxides such as SiO₂ and Al₂ O₃, and metalsulfides such as ZnS may be adopted as the protective film 24.

Referring to FIG. 2, there is shown the structure of a magneto-opticalrecording medium 12a according to a second preferred embodiment of thepresent invention, which is a modification of the first preferredembodiment shown in FIG. 1. The recording medium 12a according to thesecond preferred embodiment differs from the recording medium 12 shownin FIG. 1 in the point that a nonmagnetic intermediate layer 26 isinterposed between the magnetic reproducing layer 18 and the magneticcontrol layer 20. The nonmagnetic intermediate layer 26 is formed frommetal nitrides such as SiN and AlN, metal oxides such as SiO₂ and Al₂O₃, or metal sulfides such as ZnS.

As the nonmagnetic intermediate layer 26 is interposed between themagnetic reproducing layer 18 and the magnetic control layer 20, theexchange bond between the magnetic reproducing layer 18 and the magneticcontrol layer 20 is perfectly cut off. The nonmagnetic intermediatelayer 26 must be thin enough to permit the magnetostatic bond betweenthe magnetic control layer 20 and the magnetic reproducing layer 18 whenthe magnetic reproducing layer 18 is heated to temperatures higher thanthe given temperature. Specifically, the thickness of the nonmagneticintermediate layer 26 is preferably in the range of 1 nm to 10 nm.

FIG. 3 shows the structure of a magneto-optical recording medium 12baccording to a third preferred embodiment of the present invention,which is a modification of the second preferred embodiment shown in FIG.2. In the third preferred embodiment, the nonmagnetic intermediate layer26 is interposed between the magnetic control layer 20 and the magneticrecording layer 22.

Referring to FIG. 4, there is schematically shown the structure of amagneto-optical disk drive unit employable for the reproducing methodfor information according to the present invention. A magneto-opticaldisk 30 is formed by laminating a magnetic film 28 consisting of threelayers on the transparent substrate 14. The magneto-optical disk 30 isrotated by a spindle motor 32. Reference numeral 34 denotes anelectromagnet to be driven by an electromagnet drive circuit 36. Theelectromagnet 34 applies a bias magnetic field having a given directionto the magneto-optical disk 30. The direction of the bias magnetic fieldis changed from an upward direction to a downward direction and viceversa according to the direction of a current flowing in theelectromagnet 34.

Alternatively, the electromagnet 34 may be replaced by a compactpermanent magnet generating a magnetic field of hundreds of Oe. In thiscase, the direction of the bias magnetic field is changed by rotating Sand N poles of the magnet by 180°. A data signal to be written isgenerated from a signal generating circuit 38, and is input into a laserdrive circuit 40. The laser drive circuit 40 drives a laser diode 42with modulation according to the data signal.

A laser beam generated from the laser diode 42 is collimated by acollimator lens 44 to pass through a beam splitter 46, then beingfocused on the magnetic film 28 of the magneto-optical disk 30 by anobjective lens 48. While the bias magnetic field is being applied in agiven direction by the electromagnet 34, the laser diode 42 is driven todirect the laser beam onto the magnetic film 28 of the magneto-opticaldisk 30, thereby writing the data signal to the disk 30.

On the other hand, in reproducing information (data signal) recorded onthe magneto-optical disk 30, the laser diode 42 is driven to apply areproducing beam power to the magneto-optical disk 30 while the biasmagnetic field is being applied in a given direction by theelectromagnet 34. Reflected light from the magneto-optical disk 30 iscollimated by the objective lens 48 and is then reflected by the beamsplitter 46. The reflected light from the beam splitter 46 is allowed topass through an analyzer 50 and is then condensed by a lens 52 to reacha photodetector 54, in which the information recorded on themagneto-optical disk 30 is reproduced as an electrical signal.

An erasing method for data recorded on the magneto-optical recordingmedium according to the first preferred embodiment shown in FIG. 1 willnow be described with reference to FIG. 5. The magneto-optical recordingmedium is rotated in the direction of arrow R. In an area d of themagneto-optical recording medium, a bias magnetic field Hb is applied inan erasing direction and a laser beam 56 is directed onto the medium toheat the recording layer 22 to temperatures higher than its Curietemperature, thereby erasing the data.

In an area c being cooled, the temperature is higher than the Curietemperature of the control layer 20, and therefore no magnetizationappears in the control layer 20. In an area b, however, the temperatureis lower than the Curie temperature of the control layer 20, andtherefore the magnetization of the recording layer 22 is transferred tothe control layer 20. Further, the magnetization of the control layer 20is transferred to the reproducing layer 18. However, in an area afurther lowered in temperature, the easy direction of magnetization ofthe reproducing layer 18 is changed to an in-plane direction. Thus, adata erased condition is obtained.

A writing method for data will now be described with reference to FIG.6. In writing data to the magneto-optical recording medium, the biasmagnetic field Hb is applied in the direction opposite to that in thecase of erasing shown in FIG. 5. In an area i as a part of a laser beamirradiation area, magnetization information of the recording layer 22has already been transferred to the reproducing layer 18. An area h is ahigh-temperature area in a beam spot. In this area h, the reproducinglayer 18, the control layer 20, and the recording layer 22 are heated totemperatures higher than their Curie temperatures, thereby writing datato the recording layer 22 as downward applying the bias magnetic fieldHb.

In an area g being cooled after the writing of the data, both thereproducing layer 18 and the recording layer 22 are magnetized in thedirection of the bias magnetic field Hb. In this area g, the temperatureis higher than the Curie temperature of the control layer 20, andtherefore no magnetization appears in the control layer 20. In an area ffurther cooled, magnetization appears in the control layer 20, and thedirection of magnetization of the control layer 20 becomes identicalwith the directions of magnetization of the reproducing layer 18 and therecording layer 22. In an area e further cooled near room temperature,the magnetization condition of the recording layer 22 is unchanged, butthe direction of magnetization of the reproducing layer 18 is changedfrom the perpendicular direction to the in-plane direction. The size ofa mark to be recorded is controlled by adjusting the laser power.

A reproducing method for data recorded on the magneto-optical recordingmedium will now be described with reference to FIGS. 7A to 9B. Aspreviously described with reference to FIG. 28B, a temperature gradientis formed in the beam spot formed on the recording medium. Generally,this is due to the fact that (a) a temperature distribution similar to aGaussian distribution is formed in the focused beam spot, and (b) whenthe recording medium is moved (i.e., the beam spot is relatively moved),heat is accumulated at a trailing portion of the beam spot. An actualtemperature distribution is decided according to the reproducing laserpower, the rotating speed of the recording medium, the thickness of themagnetic film, etc.

In FIGS. 7A to 9B, Hr denotes a bias magnetic field for reproduction,and it is applied in the direction opposite to the direction of the biasmagnetic field Hb for data writing. However, as will be described later,the direction of the bias magnetic field Hr for data reproduction may bethe same as the direction of the bias magnetic field Hb for datawriting. FIGS. 7A and 7B show the condition where the temperature insidethe beam spot 58 is lower than a temperature Tcopy at which themagnetization of the recording layer 22 is transferred to thereproducing layer 18. In this condition, the direction of magnetizationof the reproducing layer 18 is an in-plane direction to form an in-planemask 60 in the reproducing layer 18, so that no magneto-optical signalis output.

FIGS. 8A and 8B show the condition where the temperature inside the beamspot 58 is higher than the temperature Tcopy and lower than the Curietemperature Tc2 of the control layer 20. In this condition, an in-planemask 60 and an opening 62 are formed in the beam spot 58. This conditionis similar to the data reproducing condition in Japanese PatentLaid-open No. 5-81717 mentioned above, and a magneto-optical signal canbe read through the opening 62. When the reproducing laser power isfurther increased, the condition shown in FIGS. 9A and 9B is obtained.That is, there are formed in the beam spot 58 a low-temperature areawhere the direction of magnetization of the reproducing layer 18 is anin-plane direction, an intermediate-temperature area where themagnetization of the recording layer 22 is transferred to the controllayer 20 and the reproducing layer 18 by exchange bond, and ahigh-temperature area where the temperature of the control layer 20becomes higher than its Curie temperature. As shown in FIG. 9A, aplurality of marks 66 shown by broken lines are formed in a track 64.

In the low-temperature area, the in-plane mask 60 is formed, while inthe high-temperature area, an up spin mask 68 is formed to upward directthe magnetization of the reproducing layer 18. The opening 62 is formedin the intermediate-temperature area between the in-plane mask 60 andthe up spin mask 68. The condition in the up spin mask 68 is that therecording medium is heated to temperatures higher than the Curietemperature Tc2 of the control layer 20 to result in the absence ofmagnetization of the control layer 20, causing no magnetic bond betweenthe reproducing layer 18 and the recording layer 22.

Accordingly, the reproducing layer 18 has almost no coercive force, andtherefore the direction of magnetization of the reproducing layer 18becomes identical with the direction of the bias magnetic field Hr forreproduction. That is, in the high-temperature area whose temperature ishigher than the Curie temperature of the control layer 20, themagnetization of the reproducing layer 18 is always upward directed, sothat the reproducing layer 18 functions as a kind of mask to hinderoutputting of a magneto-optical signal. Accordingly, the very smallopening 62 can be formed as compared with the conventional methoddescribed in Japanese Patent Laid-open No. 5-81717. Furthermore, theopening 62 is formed at the central portion of the beam spot at whichthe laser intensity is larger than that at the peripheral portion of thebeam spot, thereby obtaining a large magneto-optical signal output.

Examination on the degree of improvement in resolution was made inreproducing information recorded on the magneto-optical recording mediumaccording to the first preferred embodiment by using the reproducingmethod of the present invention. When the wavelength of the laser usedfor reproduction is set to 780 nm, the resolution (which means a marklength to be accurately reproducible) is about 0.8 μm from thediffraction limit theory. In the prior art described in Japanese PatentLaid-open No. 5-81717 wherein a reproducing layer is provided on arecording layer, the resolution is about 0.5 μm even with an optimizedreproducing power. In addition, since the opening is formed at theperipheral portion of the beam spot, the magneto-optical signal outputis small and a large reproduction output cannot be obtained in the priorart.

According to the reproducing method of the present invention, since thein-plane mask 60 and the up spin mask 68 are formed in the beam spot,the resolution is improved to about 0.3 μm. Furthermore, since theopening 62 is formed near the center of the beam spot, a magneto-opticalsignal output larger than that obtained by the conventional reproducingmethod can be obtained. FIG. 10 shows the mark length dependency of C/Nin the reproducing method according to the first preferred embodiment ofthe present invention in comparison with the conventional reproducingmethod. In FIG. 10, the reproducing method of the present invention isshown by a solid line, and the conventional reproducing method is shownby a broken line.

FIG. 11 shows the reproducing power dependency of C/N, in which when thereproducing power is higher than a certain value, e.g., 1.7 mW, C/N isremarkably improved. This is due to the fact that the reproducing powerhigher than such a certain value causes the formation of the in-planemask and the up spin mask in the beam spot, resulting in the formationof a very small opening.

In the present invention as described above, only a signal at theopening as the intermediate-temperature area is transferred from therecording layer 22 to the reproducing layer 18, and the other areaexcept the opening in the beam spot is magneto-optically masked by thein-plane mask 60 and the up spin mask 68, thereby allowing thereproduction of a magneto-optical signal in a very small area.Accordingly, the interference of signals between adjacent marks can beeliminated, and the pitch of marks can be more reduced. In addition, thecrosstalk between adjacent tracks can also be improved.

FIGS. 12A and 12B show another reproducing method of the presentinvention in the case where the bias magnetic field Hr for reproductionis applied downward, that is, it is applied in the same direction as thedirection of the bias magnetic field Hb for data recording. In thiscase, the in-plane mask 60 and a down spin mask 68' are formed in thebeam spot 58, and the small opening 62 is formed between the two masks60 and 68' in the same manner as that by the reproducing method shown inFIGS. 9A and 9B. Thus, the reproducing method of the present inventiondoes not limit the direction of the bias magnetic field forreproduction, that is, allows both the upward direction and the downwarddirection.

Referring to FIG. 13, there is shown the structure of a magneto-opticalrecording medium 12c according to a fourth preferred embodiment of thepresent invention. In the following description of this preferredembodiment, the same parts as those in the first preferred embodimentshown in FIG. 1 are denoted by the same reference numerals, and theexplanation thereof will be omitted to avoid repetition. In therecording medium 12c according to this preferred embodiment, each of amagnetic reproducing layer 18 and a magnetic control layer 20' has aneasy axis of magnetization in a plane at room temperature. The magneticcontrol layer 20' is formed from a rare earth-transition metal amorphousalloy film such as GdFe. Preferably, the easy axis of magnetization ofeach of the reproducing layer 18 and the control layer 20' is changedfrom the in-plane direction to the perpendicular direction attemperatures higher than a given temperature to which the medium isheated by a reproducing beam power.

A Curie temperature Tc1 of the reproducing layer 18, a Curie temperatureTc2 of the control layer 20', and a Curie temperature Tc3 of therecording layer 22 are related to satisfy Tc1>Tc2 and Tc3>Tc2.Preferably, a magnetic moment of rare earth metal contained in thereproducing layer 18 is predominant over a magnetic moment of transitionmetal contained in the reproducing layer 18 at room temperature. Thereproducing layer 18 contains at least Gd and Fe. The content of Gd inthe reproducing layer 18 is preferably in the range of 26 at % to 35 at%.

Similarly, it is preferable that a magnetic moment of rare earth metalcontained in the control layer 20' is predominant over a magnetic momentof transition metal contained in the control layer 20' at roomtemperature. The control layer 20' contains at least Gd and Fe, or atleast Dy and Fe. The content of Gd in the control layer 20' ispreferably in the range of 26 at % to 35 at %. Preferably, the controllayer 20' further contains a nonmagnetic material selected from thegroup consisting of Si, Al, and Ti. The content of the nonmagneticmaterial in the control layer 20' is set to preferably 60 at % or less.

An erasing method for data according to this preferred embodiment willnow be described with reference to FIG. 14. A bias magnetic field Hb isapplied downward, and a laser beam is directed onto the recording mediumto temperatures higher than a temperature at which the direction ofmagnetization of the recording layer 22 is inverted, thereby downwarddirecting the magnetization of the recording layer 22. When the laserbeam is removed, the temperature of the recording medium lowers to roomtemperature. Both the reproducing layer 18 and the control layer 20'become in-plane magnetization films at room temperature, so that therecording layer 22 is not magnetically bonded to the reproducing layer18 and the control layer 20'.

A writing method for data according to this preferred embodiment willnow be described with reference to FIG. 15. A bias magnetic field Hb isapplied in the direction opposite to the erasing direction mentionedabove, that is, in the upward direction, and a strong laser beam isdirected onto only a data writing portion of the recording medium,thereby upward directing the magnetization of the recording layer 22 atonly the data writing portion. When the laser beam is removed, thetemperature of the recording medium lowers to room temperature. Both therecording layer 18 and the control layer 20' become in-planemagnetization films at room temperature, so that the recording layer 22is not magnetically bonded to the reproducing layer 18 and the controllayer 20'.

A single-masked reproducing method according to this preferredembodiment will now be described with reference to FIGS. 16A and 16B.When the laser beam is directed onto the track 64 of the recordingmedium, there are formed in the beam spot 58 a low-temperature areawhose temperature is lower than Tcopy and a high-temperature area whosetemperature is higher than Tcopy and lower than the Curie temperatureTc2 of the control layer 20'. Accordingly, there are formed in the beamspot 58 an in-plane mask 60 where the direction of magnetization of thereproducing layer 18 is an in-plane direction and an opening 62 wherethe magnetization of the recording layer 22 is transferred to thecontrol layer 20' and the reproducing layer 18. This condition issimilar to that of data reproduction in Japanese Patent Laid-open No.5-81717 mentioned above. That is, a magneto-optical signal can be readthrough the opening 62.

When the reproducing laser power is further increased, there are formedin the beam spot 58 a low-temperature area where the direction ofmagnetization of the reproducing layer 18 is an in-plane direction, anintermediate-temperature area where the magnetization of the recordinglayer 22 is transferred to the control layer 20' and the reproducinglayer 18 by exchange bond, and a high-temperature area whose temperatureis higher than the Curie temperature of the control layer 20' as shownin FIGS. 17A and 17B. In the low-temperature area in the beam spot 58,the direction of magnetization of the control layer 20' becomes aperpendicular direction due to the exchange bond to the recording layer22; however, the easy axis of magnetization of the reproducing layer 18remains in a plane, so that the in-plane mask 60 is formed.

In the intermediate-temperature area, the magnetization of the recordinglayer 22 is transferred to the control layer 20' by exchange bond, andthe magnetization of the control layer 20' is transferred to thereproducing layer 18 by exchange bond, so that the opening 62 is formed.In the high-temperature area, the medium is heated to temperatureshigher than the Curie temperature of the control layer 20', so that themagnetization of the control layer 20' disappears to cut off themagnetic bond between the reproducing layer 18 and the recording layer22. Accordingly, the reproducing layer 18 is magnetized in the directionof the bias magnetic field Hr to form the up spin mask 68.

In the data reproducing method according to this preferred embodiment,even when the bias magnetic field Hr for reproduction is not applied,data can sometimes be reproduced. That is, in the high-temperature areawhose temperature is higher than the Curie temperature Tc2 of thecontrol layer 20', no exchange bond exists between the reproducing layer18 and the recording layer 22. When a recorded mark in the reproducinglayer 18 is very small, the direction of magnetization of thereproducing layer 18 becomes identical with the direction ofmagnetization around the mark. In other words, in the high-temperaturearea meeting T>Tc2, the mark transferred to the reproducing layer 18undergoes the magnetic influence around the mark to spontaneouslydisappear. Thus, a mask is formed in the high-temperature area even withno use of the bias magnetic field for reproduction.

Accordingly, when a magneto-optical signal is differentially detected,the low-temperature area functions as a magneto-optical mask, and thehigh-temperature area also functions as a magneto-optical mask; however,information in the recording layer 22 is transferred to the reproducinglayer 18 in the intermediate-temperature area. Thus, only theintermediate-temperature area formed between the two masks becomes anopening to allow the magnetically induced super resolution reproduction.

Referring to FIG. 18, there is shown the structure of a magneto-opticalrecording medium 12' according to a fifth preferred embodiment of thepresent invention. The magneto-optical recording medium 12' according tothis preferred embodiment differs from the magneto-optical recordingmedium 12 according to the first preferred embodiment shown in FIG. 1 inthe point that a magnetic reproduction assisting layer 17 is interposedbetween the dielectric layer 16 and the reproducing layer 18. Theassisting layer 17 has an easy axis of magnetization in theperpendicular direction like the control layer 20 and the recordinglayer 22. The reproducing layer 18 has an easy axis of magnetization ina plane at room temperature. Preferably, the easy axis of magnetizationof the reproducing layer 18 is changed to the perpendicular direction attemperatures higher than a given temperature to which the medium isheated by a reproducing beam power.

A Curie temperature Tc1 of the assisting layer 17, a Curie temperatureTc2 of the reproducing layer 18, a Curie temperature Tc3 of the controllayer 20, and a Curie temperature Tc4 of the recording layer 22 arerelated to satisfy Tc1>Tc3, Tc2>Tc3, and Tc4>Tc3, preferably,Tc1>Tc4>Tc3 and Tc2>Tc4>Tc3.

The Curie temperature of the assisting layer 17 is made high in order toenlarge a reproduction signal output. The Curie temperature of thereproducing layer 18 is made high to some extent because the reproducinglayer 18 has a large value of saturation magnetization at roomtemperature to become an in-plane magnetization film, while the layer 18must become a perpendicular magnetization film at temperatures near thetemperature to which the medium is heated by the reproducing power.

The Curie temperature of the control layer 20 is set lower than theCurie temperature of the recording layer 22 because the control layer 20has a role of cutting off an exchange bonding force between thereproducing layer 18 and the recording layer 22 at temperatures near thetemperature to which the medium is heated by the reproducing power. TheCurie temperature of the recording layer 22 is set higher than the Curietemperature of the control layer 20 in order that a recorded signal inthe recording layer 22 does not disappear even at temperatures near thetemperature to which the medium is heated by the reproducing power.

Further, a coercive force Hc1 of the assisting layer 17 at roomtemperature, a coercive force Hc2 of the reproducing layer 18 at roomtemperature, a coercive force Hc3 of the control layer 20 at roomtemperature, and a coercive force Hc4 of the recording layer 22 arerelated to satisfy Hc4>Hc3 and Hc4>Hc1.

It is desired that the coercive force of the recording layer 22 is largeat room temperature, because the direction of magnetization of therecording layer 22 must not be inverted by an external magnetic field.Accordingly, the coercive force of the recording layer 22 is the largestone of the coercive forces of all the layers. The coercive force of theassisting layer 17 is set small because the direction of magnetizationof the assisting layer 17 must be inverted by a bias magnetic field. Thecoercive force of the control layer 20 must be set smaller than thecoercive force of the recording layer 22 in order to perfectlyexchange-bond the control layer 20 and the recording layer 22 at roomtemperature.

Preferably, the control layer 20 and the recording layer 22 aremagnetically bonded so as to satisfy Hc3<σw/(2Ms3·h3), where Ms3represents the value of saturation magnetization of the control layer20, h3 represents the thickness of the control layer 20, and σwrepresents the domain wall energy between the control layer 20 and therecording layer 22. To ensure the perfect exchange bond between thecontrol layer 20 and the recording layer 22, the satisfaction of thiscondition is desirable.

FIG. 19 shows a condition where data is erased from the recording mediumaccording to the fifth preferred embodiment. That is, data is erased bydirecting a laser beam onto the magneto-optical recording medium 12' toheat the recording medium to temperatures higher than the Curietemperature of the recording layer 22, and applying a bias magneticfield Hb downward or upward. In FIG. 19, the arrows denoting thedirection of magnetization of the reproducing layer 18 are directedleftward; however, the direction of the arrows is merely illustrativefor convenience, and the direction of magnetization is not limited to aspecific direction in a plane.

FIG. 20 shows a condition where data is written on the magneto-opticalrecording medium by directing a laser beam having write power onto themedium as applying a bias magnetic field Hb in the direction opposite tothe data erasing direction. That is, the bias magnetic field Hb isapplied upward, and the recording medium is heated to temperatures nearthe Curie temperature of the recording layer 22, thereby upwarddirecting the magnetization of the recording layer 22. At this time, thecontrol layer 20 is heated to temperatures higher than its Curietemperature, so that the magnetization of the control layer 20disappears. The directions of magnetization of the assisting layer 17and the reproducing layer 18 become identical with the direction of thebias magnetic field Hb.

When the temperature of the medium lowers to a temperature less than theCurie temperature of the control layer 20, the control layer 20 isexchange-bonded to the recording layer 22, so that the magnetization ofthe control layer 20 is directed upward. When the temperature of themedium further lowers, the value of saturation magnetization of thereproducing layer 18 becomes large and the direction of magnetization ofthe layer 18 therefore becomes an in-plane direction. Accordingly, in arecording power irradiation area of the recording medium, the directionsof magnetization of all the assisting layer 17, the control layer 20,and the recording layer 22 become upward.

In an area where recording is not performed (i.e., an area irradiatedwith power corresponding to the reproducing power), the medium is heatedto temperatures higher than the Curie temperature of the control layer20 by this laser power, so that the directions of magnetization of theassisting layer 17 and the reproducing layer 18 become upward. When thetemperature of the medium lowers, the direction of magnetization of theassisting layer 17 remains upward, but the direction of magnetization ofthe reproducing layer 18 becomes an in-plane direction. Further, thedirections of magnetization of the control layer 20 and the recordinglayer 22 become downward, which corresponds to a data erased condition.

A data reproducing method according to the fifth preferred embodimentwhen applying a bias magnetic field Hr for reproduction in the upwarddirection, i.e., in the same direction as that of the bias magneticfield for recording will now be described with reference to FIGS. 21Aand 21B. As shown in FIG. 21A, a beam spot 58 is formed on the recordingmedium, and a temperature distribution is accordingly formed in the beamspot 58. In a low-temperature area where the direction of magnetizationof the reproducing layer 18 is an in-plane direction, the direction ofmagnetization of the assisting layer 17 becomes identical with theupward direction, i.e., the direction of the bias magnetic field Hr forreproduction, so that an upstream up spin mask 68a is formed in the beamspot 58.

In an intermediate-temperature area where the reproducing layer 18 isexchange-bonded to the control layer 20, the recording layer 22 and thecontrol layer 20 are exchange-bonded together, and the reproducing layer18 and the assisting layer 17 are also exchange-bonded together. As aresult, the magnetization of the recording layer 22 is transferred tothe assisting layer 17. That is, an opening 62 for data reading isformed in the beam spot 58. In a high-temperature area heated totemperatures higher than the Curie temperature of the control layer 20,the magnetization of the control layer 20 disappears, and the directionsof magnetization of the assisting layer 17 and the reproducing layer 18become upward, so that a downstream up spin mask 68b is formed in thebeam spot 58.

When the area of the medium irradiated with the laser beam is moved awayfrom the beam spot 58 to lower the temperature of the medium down totemperatures lower than the Curie temperature of the control layer 20,the direction of magnetization of the assisting layer 17 remains upward,but the direction of magnetization of the reproducing layer 18 becomesan in-plane direction. Further, the directions of magnetization of thecontrol layer 20 and the recording layer 22 become identical with thosein the recorded condition. In this manner, the low-temperature area, theintermediate-temperature area, and the high-temperature area are formed,and the opening 62 is formed in the intermediate-temperature area byappropriately controlling the reproducing power. Further, the upstreamup spin mask 68a and the downstream up spin mask 68b are formed in thelow-temperature area and the high-temperature area, respectively. As aresult, a magneto-optical signal can be read through only theintermediate-temperature area, thus allowing the magnetically inducedsuper resolution reproduction.

FIGS. 22A and 22B show a data reproducing method according to the fifthpreferred embodiment when downward applying the bias magnetic field Hrfor reproduction. As similar to the case of FIGS. 21A and 21B, themagnetically induced super resolution reproduction is allowed also inthis case. However, in this case, an upstream down spin mask 68a' and adownstream down spin mask 68b' are formed in the beam spot 58. Further,an opening 62 is similarly formed in the intermediate-temperature area.

In the magneto-optical recording media according to the first to fourthpreferred embodiments, a transition area where the direction ofmagnetization of the reproducing layer 18 is changed from the in-planedirection to the perpendicular direction at the boundary between thein-plane mask 60 and the opening 62 is relatively wide to cause anincrease in noise, with the result that the C/N of reproduction outputdoes not become so large. To the contrary, in the magneto-opticalrecording medium according to the fifth preferred embodiment shown inFIG. 18, a transition area where the upward direction of magnetizationof the assisting layer 17 for reproducing recorded information isinverted to the downward direction is narrow because the assisting layer17 is a perpendicular magnetization film, thereby reducing the noise toimprove the C/N of reproduction output.

Referring to FIG. 23, there is shown the structure of a magneto-opticalrecording medium 12d according to a sixth preferred embodiment of thepresent invention. In the following description of this preferredembodiment, the same parts as those in the first to fifth preferredembodiments mentioned above are denoted by the same reference numerals,and the explanation thereof will be omitted to avoid repetition. Amagnetic reproducing layer 18' formed from a rare earth-transition metalamorphous alloy film such as GdFeCo is laminated on the dielectric layer16. The reproducing layer 18' has an easy axis of magnetizationperpendicular to the substrate 14.

A magnetic control layer 20' formed from a rare earth-transition metalamorphous alloy film such as GdFeCo is laminated on the reproducinglayer 18'. The control layer 20' has an easy axis of magnetization in aplane at room temperature. Preferably, the easy axis of magnetization ofthe control layer 20' is changed from the in-plane direction to theperpendicular direction at temperatures higher than a given temperatureto which the medium is heated by a reproducing beam power. A Curietemperature Tc1 of the reproducing layer 18', a Curie temperature Tc2 ofthe control layer 20', and a Curie temperature Tc3 of the recordinglayer 22 are related to satisfy Tc1>Tc2 and Tc3>Tc2.

Further, a coercive force Hc1 of the reproducing layer 18' at roomtemperature and a coercive force Hc3 of the recording layer 22 at roomtemperature are related to satisfy Hc3>Hc1. The reproducing layer 18'may be formed from an amorphous alloy film containing Tb, Gd, Fe, andCo, and the control layer 20' may be formed from an amorphous alloy filmcontaining Gd and Fe. Preferably, the control layer 20' further containsa nonmagnetic material selected from the group consisting of Si, Al, andTi.

A data erasing method according to this preferred embodiment will now bedescribed with reference to FIG. 24. A bias magnetic field Hb is applieddownward, and a laser beam is directed onto the recording medium to heatthe medium to temperatures higher than the Curie temperature of therecording layer 22, thereby downward directing the magnetization of therecording layer 22. When the laser beam is removed, the temperature ofthe recording medium lowers to room temperature. At room temperature,the control layer 20' becomes an in-plane magnetization film, and thereproducing layer 18' is not magnetically bonded to the recording layer22. Accordingly, the direction of magnetization of the reproducing layer18' is made downward by a small magnetic field like an erasing biasmagnetic field.

A data writing method according to this preferred embodiment will now bedescribed with reference to FIG. 25. The bias magnetic field Hb isapplied in the direction opposite to the erasing direction, i.e., in theupward direction, and a strong laser beam is directed onto only arecording portion of the medium, thereby upward directing themagnetization of the recording layer 22 at only the recording portion.When the laser beam is removed, the temperature of the recording mediumlowers to room temperature. At room temperature, the control layer 20'becomes an in-plane magnetization film, and the reproducing layer 18' isnot magnetically bonded to the recording layer 22. Accordingly, thedirection of magnetization of the reproducing layer 18' is made upwardby a small magnetic field like a bias magnetic field.

A single-masked reproducing method according to this preferredembodiment will now be described with reference to FIGS. 26A and 26B.There are formed in the beam spot 58 directed onto the track 64 alow-temperature area whose temperature is lower than Tcopy and ahigh-temperature area whose temperature is higher than Tcopy and lowerthan the Curie temperature Tc2 of the control layer 20'. An up spin mask60 is formed in the low-temperature area in the beam spot 58, and anopening 62 is formed in the high-temperature area in the beam spot 58.This condition is similar to the data reproduction condition in JapanesePatent Laid-open No. 5-81717 mentioned above, and a magneto-opticalsignal can be read through the opening 62.

When the reproducing laser power is further increased, there are formedin the beam spot 58 a low-temperature area where the direction ofmagnetization of the reproducing layer 18' is identical with thedirection of the reproducing bias magnetic field Hr, anintermediate-temperature area where the magnetization of the recordinglayer 22 is transferred to the control layer 20' and the reproducinglayer 18' by exchange bond, and a high-temperature area whosetemperature is higher than the Curie temperature Tc2 of the controllayer 20' as shown in FIGS. 27A and 27B. In the low-temperature area andthe high-temperature area, up spin masks 60 and 68 where the directionof magnetization of the reproducing layer 18' is made identical with thedirection of the bias magnetic field Hr are formed, respectively.

In the up spin mask 68, the recording medium is heated to temperatureshigher than the Curie temperature Tc2 of the control layer 20', so thatthe magnetization of the control layer 20' disappears and thereproducing layer 18' is not magnetically bonded to the recording layer22. Accordingly, the direction of magnetization of the reproducing layer18' is made identical with the direction of magnetization of the biasmagnetic field Hr because of its small coercive force at roomtemperature. That is, in the high-temperature area whose temperature ishigher than the control layer 20', the magnetization of the reproducinglayer 18' is directed always upward, so that the reproducing layer 18'functions as a kind of mask to hinder outputting of a magneto-opticalsignal. Accordingly, the opening 62 having a very small size as comparedwith that obtained by the conventional method described in JapanesePatent Laid-open No. 5-81717 can be formed. In addition, since theopening 62 is formed at the central portion of the beam spot larger inlaser intensity than at the peripheral portion of the beam spot, a largemagneto-optical signal output can be obtained.

EXAMPLE 1

Targets of TbFeCo, TbFe, GdFeCo, and Si and a polycarbonate substratehaving a track pitch of 1.3 μm were set in a sputtering device, and achamber of the sputtering device was evacuated to 10⁻⁵ Pa. Then, asilicon nitride (SiN) film having a thickness of 70 nm was formed on thesubstrate by DC sputtering under the following conditions. This filmserves not only to protect the magnetic film from oxidation, but also toexhibit an enhance effect such that a magneto-optical signal isenhanced.

gas pressure: 0.2 Pa

sputter gas: Ar, N₂

pressure ratio: Ar:N₂ =7:3

applied power: 0.8 kW

Then, the chamber was evacuated to 10⁻⁵ Pa again, and the films ofGdFeCo, TbFe, and TbFeCo were continuously formed in this order on theSiN film by DC sputtering under the following conditions.

gas pressure: 0.5 Pa

sputter gas: Ar

applied power: 1 kW

The composition, thickness, and magnetic characteristics of eachmagnetic layer are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                       Thick-          Compen-                                                       ness    Curie   sation                                         Composition    (nm)    Temp.   Temp.  Dominant                                ______________________________________                                        Reproducing                                                                           Gd.sub.29 Fe.sub.55 Co.sub.16                                                            40      330° C.                                                                      220° C.                                                                       RE rich                               Layer                                                                         Control Tb.sub.17 Fe.sub.83                                                                       8      140° C.                                                                      --     TM rich                               Layer                                                                         Recording                                                                             Tb.sub.19 Fe.sub.73 Co.sub.8                                                             40      220° C.                                                                      --     TM rich                               Layer                                                                         ______________________________________                                    

In Table 1, RE represents rare earth, and TM represents transitionmetal. Further, a silicon nitride film having a thickness of 100 nm wasformed as a protective layer on the recording layer by a method similarto the above method. This silicon nitride film serves to protect themagnetic film from oxidation.

The recording characteristic of the magneto-optical disk thus producedwas examined by using the drive unit shown in FIG. 4. The wavelength oflaser used is 780 nm.

First, all data recorded on the magneto-optical disk were erased, thatis, the magneto-optical disk was initialized. At this time, the laserpower was set to 9 mW, and a bias magnetic field was applied upward. Therecording of information was performed by applying a bias magnetic fieldin the direction opposite to that in the initialization, i.e., in thedownward direction as rotating the disk at a linear velocity of 3 m/secand directing a laser beam onto the disk with a recording power of 4 mW,a frequency of 7.5 MHz, and an emission duty ratio of 26%. As theresult, a mark having a length of about 0.4 μm was recorded on the disk.

The reproducing characteristic of the magneto-optical disk was nextexamined. In this case, a bias magnetic field for reproduction wasapplied upward. With a reproducing power of 1.5 mW, no magneto-opticalsignal output for a previously recorded signal was obtained. This isconsidered to be due to the fact that an in-plane mask was formed in thewhole area in the beam spot. With a reproducing power of 1.6 mW, themagnetization of the recording layer 22 was transferred through thecontrol layer 20 to the reproducing layer 18 to obtain a magneto-opticalsignal output. This is considered to be due to the fact that an areahaving temperatures higher than the temperature at which themagnetization of the recording layer 22 is transferred to thereproducing layer 18 was formed in the beam spot to form the in-planemask 60 and the opening 62. A signal-to-noise ratio (C/N) at this timewas 35 dB.

With a reproducing power of 1.7 mW, there were formed in the beam spot alow-temperature area where the direction of magnetization of thereproducing layer 18 is an in-plane direction, anintermediate-temperature area where the magnetization of the recordinglayer 22 is transferred through the control layer 20 to the reproducinglayer 18 by exchange bond, and a high-temperature area whose temperatureis higher than the Curie temperature of the control layer 20. In thelow-temperature area, the in-plane mask 60 was formed. In thehigh-temperature area, the direction of magnetization of the reproducinglayer 18 was made identical with the direction of the bias magneticfield, i.e., the upward direction, to form the up spin mask 68 in thebeam spot. Further, the opening 62 allowing a magneto-optical signal tobe output was formed between the in-plane mask 60 and the up spin mask68 in the beam spot. As the result, a C/N value of 42 dB was obtained.

EXAMPLE 2

A magneto-optical disk having the same structure as that in Example 1except the composition of the control layer 20 was prepared by the samemethod as that in Example 1. The composition of the control layer 20 wasset to Dy₃₀ Fe₇₀. The control layer 20 has a Curie temperature of about150° C., and it is an in-plane magnetization film at room temperature.The recording characteristic of this magneto-optical disk was examined.Measurement was made under the same conditions as those in Example 1. Asthe result, with a reproducing power of 1.7 mW, there were formed in thebeam spot a low-temperature area, an intermediate-temperature area, anda high-temperature area whose temperature is higher than the Curietemperature of the control layer 20.

In the low-temperature area, the in-plane mask 60 was formed. In thehigh-temperature area, the direction of magnetization of the reproducinglayer 18 was made identical with the direction of the bias magneticfield, i.e., the upward direction, to form the up spin mask 68 in thebeam spot. Further, the opening 62 allowing a magneto-optical signal tobe output was formed between the in-plane mask 60 and the up spin mask68 in the beam spot. As the result, a C/N value of 42 dB was obtained.

EXAMPLE 3

Plural magneto-optical disks were prepared by changing the thickness ofan SiN film in the range of 1 nm to 10 nm as the nonmagneticintermediate layer 26 interposed between the reproducing layer 18 andthe control layer 20 of the magneto-optical disk having the samestructure as that of the medium in Example 1. Measurement of therecording characteristic was made under the same conditions as those inExample 1. As the result, it was found that when the intermediate layerhaving the thickness in the above range was used, the reproducing layer18 and the control layer 20 were magnetostatically bonded together by aleaked magnetic field of the recording layer 22. As similar to Example1, no magneto-optical signal output was obtained with a reproducingpower of 1.5 mW. With a reproducing power of 1.6 mW, an opening wasformed at the downstream portion in the beam spot to obtain 37 dB as amagneto-optical signal output. With a reproducing power of 1.8 mW, 47 dBwas obtained. Further, also when the nonmagnetic intermediate layer 26having a thickness in the range of 1 nm to 10 nm was interposed betweenthe control layer 20 and the recording layer 22, a magneto-opticalsignal output of 47 dB was obtained with a reproducing power of 1.8 mW.

EXAMPLE 4

Targets of TbFeCo, GdFeCo, GdFe, and Si and a polycarbonate substratehaving a track pitch of 1.2 μm were set in a sputtering device, and achamber of the sputtering device was evacuated to 10⁻⁵ Pa. Then, asilicon nitride (SiN) film having a thickness of 70 nm was formed on thesubstrate by DC sputtering under the following conditions. This filmserves not only to protect the magnetic film from oxidation, but also toexhibit an enhance effect such that a magneto-optical signal isenhanced.

gas pressure: 0.3 Pa

sputter gas: Ar, N₂

pressure ratio: Ar:N₂ =6:4

applied power: 0.8 kW

Then, the chamber was evacuated to 10⁻⁵ Pa again, and the films ofGdFeCo, GdFe, and TbFeCo were continuously formed in this order on theSiN film by DC sputtering under the following conditions.

gas pressure: 0.5 Pa

sputter gas: Ar

applied power: 1 kW

The composition, thickness, and magnetic characteristics of eachmagnetic layer are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                       Thick-          Compen-                                                       ness    Curie   sation                                         Composition    (nm)    Temp.   Temp.  Dominant                                ______________________________________                                        Reproducing                                                                           Gd.sub.39 Fe.sub.37 Co.sub.17                                                            40      300° C.                                                                      --     RE rich                               Layer                                                                         Control Gd.sub.30 Fe.sub.70                                                                      12      190° C.                                                                      --     RE rich                               Layer                                                                         Recording                                                                             Tb.sub.19 Fe.sub.73 Co.sub.8                                                             50      220° C.                                                                      --     TM rich                               Layer                                                                         ______________________________________                                    

Further, a silicon nitride film having a thickness of 100 nm was formedas a protective layer on the recording layer by a method similar to theabove method. This silicon nitride film serves to protect the magneticfilm from oxidation.

The recording characteristic of the magneto-optical disk thus producedwas examined by using the drive unit shown in FIG. 4. The wavelength oflaser used is 780 nm. First, all data recorded on the magneto-opticaldisk were erased, that is, the magneto-optical disk was initialized. Atthis time, the laser power was set to 9 mW, and a bias magnetic fieldwas applied downward. The recording of information was performed byapplying a bias magnetic field in the direction opposite to that in theinitialization, i.e., in the upward direction as rotating the disk at alinear velocity of 3 m/sec and directing a laser beam onto the disk witha recording power of 4 mW, a frequency of 7.5 MHz, and an emission dutyratio of 26%. As the result, a mark having a length of about 0.4 μm wasrecorded on the disk.

The reproducing characteristic of the magneto-optical disk was nextexamined. In this case, a bias magnetic field for reproduction wasapplied downward. The magnitude of the bias magnetic field forreproduction was set to 300 Oe. With a reproducing power of 1.5 mW, nomagneto-optical signal output for a previously recorded signal wasobtained. This is considered to be due to the fact that an in-plane maskwas formed in the whole area in the beam spot. With a reproducing powerof 1.6 mW, the magnetization of the recording layer 22 was transferredthrough the control layer 20' to the reproducing layer 18 to obtain amagneto-optical signal output. This is considered to be due to the factthat an area having temperatures higher than the temperature at whichthe magnetization of the recording layer 22 is transferred through thecontrol layer 20' to the reproducing layer 18 was formed in the beamspot to form the in-plane mask 60 and the opening 62. A signal-to-noiseratio (C/N) at this time was 42 dB.

With a reproducing power of 1.7 mW, the direction of magnetization ofthe reproducing layer 18 in a low-temperature area in the beam spot wasmade identical with an in-plane direction, and the diameter of an area(opening) where the reproducing layer 18 and the control layer 20' areexchange-bonded to the recording layer 22 became about 0.4 μm, therebyobtaining a C/N value of 45 dB. When the reproducing power was increasedto 2 mW, there were formed in the beam spot a low-temperature area wherethe direction of magnetization of the reproducing layer 18 is anin-plane direction, an intermediate-temperature area where themagnetization of the recording layer 22 is transferred through thecontrol layer 20' to the reproducing layer 18 by exchange bond, and ahigh-temperature area whose temperature is higher than the Curietemperature of the control layer 20'. In the low-temperature area, thein-plane mask 60 was formed. In the high-temperature area, the directionof magnetization of the reproducing layer 18 was made identical with thedirection of the bias magnetic field, i.e., the downward direction, toform the down spin mask 68' in the beam spot. Further, the opening 62allowing a magneto-optical signal to be output was formed between thein-plane mask 60 and the down spin mask 68' in the beam spot. As theresult, a C/N value of 47 dB was obtained.

A change in C/N with respect to the reproducing magnetic field was nextexamined. As the result, even when no reproducing magnetic field wasapplied, a C/N value of 46.5 dB was obtained. This is considered to bedue to the fact that an area of the reproducing layer 18 (magnetizationinverted area) to which the mark in the recording layer 22 had beentransferred was heated to disappear in the high-temperature area whosetemperature was higher than the Curie temperature of the control layer20'. This is considered to be caused by a small magnetic anisotropy ofGdFeCo used for the reproducing layer 18.

EXAMPLE 5

The composition of the reproducing layer 18 was examined by determiningwhether RE rich (which means that the magnetic moment of rare earthmetal is predominant over the magnetic moment of transition metal) or TMrich (which means that the magnetic moment of transition metal ispredominant over the magnetic moment of rare earth metal) is suitablefor the composition of the reproducing layer 18. In this test, pluralmagneto-optical disks were prepared by changing X in Gd_(x)(FeCo)_(100-x) as the composition of the reproducing layer 18 andforming the other magnetic layers under the same conditions as those inExample 4. Then, the reproducing characteristics of thesemagneto-optical disks were examined. Evaluation of the reproducingcharacteristics were made under the same conditions as those in Example4. The results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        X     Dominant     C/N (Hr: 0)                                                                             C/N (Hr: 300 Oe)                                 ______________________________________                                        25    TM           25        35                                               26    RE           43        45                                               30    RE           46.5      47                                               35    RE           44        45                                               37    RE           35        37                                               ______________________________________                                    

As apparent from Table 3, the content of Gd in the composition of GdFeCofor the reproducing layer 18 is preferably in the range of 26 at % to 35at %. Further, it was found that RE rich is effective for thecomposition of the reproducing layer 18.

EXAMPLE 6

The composition of the reproducing layer 18 was examined. A TbFeCo filmor a DyFeCo film used as a reproducing layer in a usual magneto-opticaldisk did not satisfy the conditions required for executing the principleof the present invention. That is, the composition providing a changefrom an in-plane magnetization film to a perpendicular magnetizationfilm at temperatures near 100° C. was not found in such an amorphousalloy film. Accordingly, GdFeCo is suitable for the composition of thereproducing layer 18.

EXAMPLE 7

The composition of the control layer 20' was examined. Firstly, thetemperature at which an in-plane magnetization film is changed to aperpendicular magnetization film was examined by changing X in Gd_(x)Fe_(100-x) as the composition of the control layer 20'. The results areshown in Table 4. In this test, the magnetic film was formed on a quartzglass by sputtering. The forming conditions are as follows:

gas pressure: 0.5 Pa

sputter gas: Ar

applied power: 1 kW

                  TABLE 4                                                         ______________________________________                                        X           24   25      28   30      35   37                                 Tp(° C.)                                                                         -30    30      70  100     150  180                                 ______________________________________                                    

As apparent from Table 4, the content of Gd in the composition of GdFefor the control layer 20' is preferably in the range of 26 at % to 35 at%. Secondly, rare earth metals to be used in the control layer 20' wereexamined. As the result, the Curie temperature of TbFe was near 150° C.,and the composition providing a change from an in-plane magnetizationfilm to a perpendicular magnetization film at temperatures near 100° C.was not found in TbFe.

The Curie temperature of DyFe was lower than 100° C.; however, the Curietemperature of Dy₂₃ Gd₇ Fe₇₀ was near 130° C. and this compositionprovided a change from an in-plane magnetization film to a perpendicularmagnetization film at temperatures near 80° C. Thus, it was found thatthis composition is suitable for a magnetic super resolutionmagneto-optical disk. In summary, it was found that it is very effectiveto contain Gd and Fe, or Dy and Fe as the composition of the controllayer 20'. Next, a nonmagnetic metal was added to GdFe as thecomposition of the control layer 20'. The addition of the nonmagneticmetal is intended to lower the Curie temperature of the control layer20' and thereby narrow the temperature range of the opening where themagnetization of the recording layer 22 is transferred through thecontrol layer 20' to the reproducing layer 18.

EXAMPLE 8

Plural magneto-optical disks were prepared by the same method as that inExample 1 with the composition of the control layer 20' set to (Gd₃₀Fe₇₀)₈₀ Ng₂₀, where Ng represents nonmagnetic metals. Thecharacteristics of the magneto-optical disks are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        Ng         Si    Ti           Al  Cu                                          C/N        48    47           47  39                                          ______________________________________                                    

As apparent from Table 5, it is effective to add the nonmagneticmaterial to the control layer 20'.

EXAMPLE 9

The amount of Si to be added to the control layer 20' was examined. Thecompositions and thicknesses of the reproducing layer 18, the controllayer 20', and the recording layer 22 were set to the same as those inExample 4. That is, the composition of the control layer 20' was set toGd₃₀ Fe₇₀. In this example, Si was added to the control layer 20' havingthis composition. The amount of Si to be added was changed by changingthe number of Si chips to be placed on a GdFe target for the controllayer 20'. The results are shown in Table 6.

                  TABLE 6                                                         ______________________________________                                        Si (at %)                                                                             0     5      10  20    30  40    50  60    70                         C/N    47    47      47  48    48  49    49  48    40                         ______________________________________                                    

As apparent from Table 6, when the content of Si is in the range of 0 at% to 60 at %, high C/N values can be obtained. When the content of Si is70 at % or higher, C/N is decreased because the exchange bonding forcebetween the reproducing layer 18 and the recording layer 22 is small.

EXAMPLE 10

Targets of TbFeCo, TbFe, first GdFeCo, second GdFeCo, and Si and apolycarbonate substrate having a track pitch of 1.2 μm were set in asputtering device, and a chamber of the sputtering device was evacuatedto 10⁻⁵ Pa. Then, a silicon nitride (SiN) film having a thickness of 70nm was formed on the substrate by DC sputtering under the followingconditions. This film serves not only to protect the magnetic film fromoxidation, but also to exhibit an enhance effect such that amagneto-optical signal is enhanced.

gas pressure: 0.2 Pa

sputter gas: Ar, N₂

pressure ratio: Ar:N₂ =7:3

applied power: 0.8 kW

Then, the chamber was evacuated to 10⁻⁵ Pa again, and the films of firstGdFeCo, second GdFeCo, TbFe, and TbFeCo were continuously formed in thisorder on the SiN film by DC sputtering under the following conditions.

gas pressure: 0.5 Pa

sputter gas: Ar

applied power: 1 kW

The composition, thickness, and magnetic characteristics of eachmagnetic layer are shown in Table 7.

                  TABLE 7                                                         ______________________________________                                                       Thick-                                                                        ness    Curie          Coercive                                Composition    (nm)    Temp.   Dominant                                                                             Force                                   ______________________________________                                        Assisting                                                                             Gd.sub.20 Fe.sub.54 Co.sub.26                                                            40      360° C.                                                                      TM rich                                                                              200 Oe                                Layer                                                                         Reproducing                                                                           Gd.sub.39 Fe.sub.37 Co.sub.24                                                            12      330° C.                                                                      RE rich                                                                              --                                    Layer                                                                         Control Tb.sub.17 Fe.sub.83                                                                      10      140° C.                                                                      TM rich                                                                               2 kOe                                Layer                                                                         Recording                                                                             Tb.sub.19 Fe.sub.73 Co.sub.8                                                             40      220° C.                                                                      TM rich                                                                               15 kOe                               Layer                                                                         ______________________________________                                    

The coercive forces Hc1, Hc2, Hc3, and Hc4 of the assisting layer, thereproducing layer, the control layer, and the recording layer arerelated to satisfy Hc4>Hc1 and Hc4>Hc3. Further, a silicon nitride filmhaving a thickness of 100 nm was formed as the protective film 24 on therecording layer 22 by DC sputtering.

The recording characteristic of the magneto-optical disk thus producedwas examined. The wavelength of laser used is 780 nm. First, all datarecorded on the magneto-optical disk were erased, that is, themagneto-optical disk was initialized. At this time, the laser power wasset to 9 mW, and a bias magnetic field was applied downward. Therecording of information was performed by upward applying a biasmagnetic field for recording as rotating the disk at a linear velocityof 3 m/sec and directing a laser beam onto the disk with a recordingpower of 4 mW, a frequency of 7.5 MHz, and an emission duty ratio of26%. As the result, a mark having a length of about 0.4 μm was recordedon the disk.

The reproducing characteristic of the magneto-optical disk was nextexamined. In this case, a bias magnetic field for reproduction wasapplied upward. With a reproducing power of 1.5 mW, no magneto-opticalsignal output for a previously recorded signal was obtained. This isconsidered to be due to the fact that an up spin mask was formed in thewhole area of the assisting layer 17 in the beam spot. With areproducing power of 1.6 mW, the magnetization of the recording layer 22was transferred through the control layer 20 and the reproducing layer18 to the assisting layer 17 to obtain a magneto-optical signal output.This is considered to be due to the fact that anintermediate-temperature area where the magnetization of the recordinglayer 22 is transferred to the reproducing layer 18 was formed in thebeam spot to form an up spin mask and an opening. A signal-to-noiseratio (C/N) at this time was 44 dB.

With a reproducing power of 1.7 mW, there were formed in the beam spot alow-temperature area where the direction of magnetization of thereproducing layer 18 is an in-plane direction, anintermediate-temperature area where the magnetization of the recordinglayer 22 is transferred through the control layer 20 and the reproducinglayer 18 to the assisting layer 17 by exchange bond, and ahigh-temperature area whose temperature is higher than the Curietemperature of the control layer 20. In the low-temperature area and thehigh-temperature area, the direction of magnetization of the assistinglayer 17 was made identical with the direction of the bias magneticfield, i.e., the upward direction, to form the upstream up spin mask 68aand the downstream up spin mask 68b in the beam spot, respectively.Further, the opening 62 allowing a magneto-optical signal to be outputwas formed between the upstream up spin mask 68a and the downstream upspin mask 68b in the beam spot. As the result, a C/N value of 49 dB wasobtained.

EXAMPLE 11

A magneto-optical disk was prepared by changing the composition andCurie temperature of the assisting layer 17 of the magneto-optical diskin Example 10 and unchanging the other conditions. That is, thecomposition of the assisting layer 17 was changed to Gd₂₃ F₅₃ Co₁₉, andthe Curie temperature of the assisting layer 17 was changed to 300° C.The reproducing characteristic of the magneto-optical disk thus preparedwas measured under the same conditions as those in Example 10 to obtainan output of 49 dB with a reproducing power of 1.8 mW.

EXAMPLE 12

The composition of the reproducing layer 18' was examined. Targets ofTbFeCo, first GdFeCo, second GdFeCo, and Si and a polycarbonatesubstrate having a track pitch of 1.2 μm were set in a sputteringdevice, and a chamber of the sputtering device was evacuated to 10⁻⁵ Pa.Then, a silicon nitride (SiN) film having a thickness of 70 nm wasformed on the substrate by DC sputtering under the following conditions.This film serves not only to protect the magnetic film from oxidation,but also to exhibit an enhance effect such that a magneto-optical signalis enhanced.

gas pressure: 0.3 Pa

sputter gas: Ar, N₂

pressure ratio: Ar:N₂ =6:4

applied power: 0.8 kW

Then, the chamber was evacuated to 10⁻⁵ Pa again, and the films of firstGdFeCo, second GdFeCo, and TbFeCo were continuously formed in this orderon the SiN film by DC sputtering under the following conditions.

gas pressure: 0.5 Pa

sputter gas: Ar

applied power: 1 kW

The composition, thickness, and magnetic characteristics of eachmagnetic layer are shown in Table 8.

                  TABLE 8                                                         ______________________________________                                                       Thick-          Compen-                                                       ness    Curie   sation                                         Composition    (nm)    Temp.   Temp.  Dominant                                ______________________________________                                        Reproducing                                                                           Gd.sub.20 Fe.sub.54 Co.sub.26                                                            40      360° C.                                                                      --     TM rich                               Layer                                                                         Control Gd.sub.39 Fe.sub.56 Co.sub.5                                                             12      210° C.                                                                      --     RE rich                               Layer                                                                         Recording                                                                             Tb.sub.19 Fe.sub.73 Co.sub.8                                                             50      220° C.                                                                      --     TM rich                               Layer                                                                         ______________________________________                                    

Further, a silicon nitride film having a thickness of 100 nm was formedas a protective layer on the recording layer by a method similar to theabove method. This silicon nitride film serves to protect the magneticfilm from oxidation.

The recording characteristic of the magneto-optical disk thus producedwas examined by using the drive unit shown in FIG. 4. The wavelength oflaser used is 780 nm. First, all data recorded on the magneto-opticaldisk were erased, that is, the magneto-optical disk was initialized. Atthis time, the laser power was set to 9 mW, and a bias magnetic fieldwas applied downward. The recording of information was performed byapplying a bias magnetic field in the direction opposite to that in theinitialization, i.e., in the upward direction as rotating the disk at alinear velocity of 3 m/sec and directing a laser beam onto the disk witha recording power of 4 mW, a frequency of 7.5 MHz, and an emission dutyratio of 26%. As the result, a mark having a length of about 0.4 μm wasrecorded on the disk.

The reproducing characteristic of the magneto-optical disk was nextexamined. In this case, a bias magnetic field for reproduction wasapplied upward. With a reproducing power of 1.5 mW, no magneto-opticalsignal output for a previously recorded signal was obtained. This isconsidered to be due to the fact that an up spin mask was formed in thewhole area of the reproducing layer 18' in the beam spot. With areproducing power of 1.6 mW, the magnetization of the recording layer 22was transferred through the control layer 20' to the reproducing layer18' to obtain a magneto-optical signal output. This is considered to bedue to the fact that an area having temperatures higher than thetemperature at which the magnetization of the recording layer 22 istransferred through the control layer 20' to the reproducing layer 18'was formed in the beam spot to form an up spin mask and an opening. Asignal-to-noise ratio (C/N) at this time was 42 dB.

With a reproducing power of 1.7 mW, the direction of magnetization ofthe recording layer 22 was made identical with the direction of the biasmagnetic field, i.e., the upward direction, and the diameter of an area(opening) where the control layer 20' is exchange-bonded to therecording layer 22 became about 0.4 μm to obtain a C/N value of 46 dB.Further, examination was made as to whether any other compositions forthe reproducing layer 18' is satisfactory for use. When TbFeCo or DyFeCowas used as the composition of the reproducing layer 18', themagnetization of the reproducing layer 18' could not be initialized bythe bias magnetic field because of a large coercive force of the abovematerial. As a result, super resolution reproduction could not beeffected.

EXAMPLE 13

In Example 12, a GdFeCo film was used as the reproducing layer 18'.However, the composition margin of the GdFeCo film showing perpendicularmagnetic anisotropy is not wide, and composition control is sometimesdifficult. From this point of view, it was tried to add a small amountof Tb capable of increasing the perpendicular magnetic anisotropy toGdFeCo in Example 13. The test was performed in the following manner.Targets of TbFeCo, TbGdFeCo, GdFeCo, and Si and a polycarbonatesubstrate having a track pitch of 1.2 μm were set in a sputteringdevice, and a chamber of the sputtering device was evacuated to 10⁻⁵ Pa.Then, a silicon nitride (SiN) film having a thickness of 70 nm wasformed on the substrate by DC sputtering under the following conditions.This film serves not only to protect the magnetic film from oxidation,but also to exhibit an enhance effect such that a magneto-optical signalis enhanced.

gas pressure: 0.3 Pa

sputter gas: Ar, N₂

pressure ratio: Ar:N₂ =6:4

applied power: 0.8 kW

Then, the chamber was evacuated to 10⁻⁵ Pa again, and the films ofTbGdFeCo, GdFeCo, and TbFeCo were continuously formed in this order onthe SiN film by DC sputtering under the following conditions.

gas pressure: 0.5 Pa

sputter gas: Ar

applied power: 1 kw

The composition, thickness, and magnetic characteristics of eachmagnetic layer are shown in Table 9.

                  TABLE 9                                                         ______________________________________                                                                        Com-                                                          Thick-          pen-                                                          ness    Curie   sation                                        Composition     (nm)    Temp.   Temp. Dominant                                ______________________________________                                        Reproducing                                                                           Tb.sub.2 Gd.sub.18 Fe.sub.54 Co.sub.26                                                    40      350° C.                                                                      --    TM rich                               Layer                                                                         Control Gd.sub.39 Fe.sub.56 Co.sub.5                                                              12      210° C.                                                                      --    RE rich                               Layer                                                                         Recording                                                                             Tb.sub.19 Fe.sub.73 Co.sub.8                                                              50      220° C.                                                                      --    TM rich                               Layer                                                                         ______________________________________                                    

Further, a silicon nitride film having a thickness of 100 nm was formedas a protective layer on the recording layer by a method similar to theabove method.

This silicon nitride film serves to protect the magnetic film fromoxidation.

The reproducing characteristic of the magneto-optical disk was examinedunder the same conditions as those in Example 12. In this case, a biasmagnetic field for reproduction was applied upward. With a reproducingpower of 1.4 mW, no magneto-optical signal output for a previouslyrecorded signal was obtained. This is considered to be due to the factthat an up spin mask was formed in the whole area of the reproducinglayer 18' in the beam spot. With a reproducing power of 1.5 mW, themagnetization of the recording layer 22 was transferred through thecontrol layer 20' to the reproducing layer 18' to obtain amagneto-optical signal output. This is considered to be due to the factthat an area having temperatures higher than the temperature at whichthe magnetization of the recording layer 22 is transferred through thecontrol layer 20' to the reproducing layer 18' was formed in the beamspot to form an up spin mask and an opening. A signal-to-noise ratio(C/N) at this time was 42 dB.

With a reproducing power of 1.6 mW, the direction of magnetization ofthe reproducing layer 18' was made identical with the direction of thebias magnetic field, i.e., the upward direction, and the diameter of anarea (opening) where the control layer 20' is exchange-bonded to therecording layer 22 became about 0.4 μm to obtain a C/N value of 46 dB.This C/N value is the same as that in Example 12. This is considered tobe due to the fact that the addition of Tb to the reproducing layer 18'causes an increase in magnetic anisotropy to reduce the noise andsimultaneously causes a decrease in magneto-optical effect to thecontrary. Thus, these contrary effects were considered to counterbalanceeach other and obtain the same result as that in Example 12. However,the composition margin of the reproducing layer 18' can be widened byadding Tb to the layer 18'.

EXAMPLE 14

The composition of the control layer 20' was examined by preparingplural magneto-optical recording media each having three magnetic layerswith the composition of the control layer 20' changed to TbFeCo, DyFeCo,and GdFeCo each showing in-plane magnetization. The recording andreproducing characteristics of these magneto-optical recording mediawere examined under the same conditions as those in Example 12. Theresults are shown in Table 10.

                  TABLE 10                                                        ______________________________________                                        Composition                                                                             TbFeCo       DyFeCo  GdFeCo                                         C/N       36           38      46                                             ______________________________________                                    

As apparent from Table 10, when TbFeCo or DyFeCo is used as thecomposition of the control layer 20', the recording and reproducingcharacteristics are poor. To the contrary, GdFeCo is suitable for thecomposition of the control layer 20'.

Further, the ratio of Gd, Fe, and Co suitable for the composition of thecontrol layer 20' was examined. That is, the reproducing characteristicwas measured by the same method as that in Example 12 when changing thecontent of Co in the composition of GdFeCo for the control layer 20'.The results are shown in Table 11.

                  TABLE 11                                                        ______________________________________                                        Co content (at %)                                                                       21      15    10      5   3     2   0                               C/N       44      45    45     46  49    49  49                               ______________________________________                                    

As apparent from Table 11, when the content of Co is in the range of 0at % to 5 at %, C/N is improved. This is considered to be due to thefact that the Curie temperature of the control layer 20' was decreasedto cause the formation of a mask at the downstream portion of the beamspot in addition to a mark at the upstream portion, with the result thata double mask was formed to improve the resolution.

EXAMPLE 15

It was found from Example 14 that the decrease in the Curie temperatureof the control layer 20' causes the formation of the double mask toimprove C/N. In Example 15, a nonmagnetic metal was added to the controllayer 20' in order to decrease the Curie temperature of the controllayer 20'. Plural magneto-optical recording media were prepared byadding various nonmagnetic metals Ng to the control layer 20' having thecomposition of GdFeCo. The reproducing characteristics of theserecording media were measured by the same method as that in Example 12.The results are shown in Table 12.

                  TABLE 12                                                        ______________________________________                                        Ng         Si    Ti           Al  Cu                                          C/N        49    48           49  39                                          ______________________________________                                    

As apparent from Table 12, it is effective to add the nonmagneticmaterial to the control layer 20'. Next, examination was made on achange in reproducing characteristic with a change in amount of Si to beadded as an example of the nonmagnetic material to the control layer20'. The compositions and thicknesses of the reproducing layer 18', thecontrol layer 20', and the recording layer 22 were set to the same asthose in Example 12. That is, the composition of the control layer 20'was set to Gd₃₉ Fe₅₅ Co₅. In this example, Si was added to thiscomposition of the control layer 20'. The amount of Si to be added waschanged by changing the number of Si chips to be placed on a GdFeCotarget for the control layer 20'. The results are shown in Table 13.

                  TABLE 13                                                        ______________________________________                                        Si (at %)                                                                             0     5      10  20    30  40    50  60    70                         C/N    46    46      47  48    48  49    49  48    40                         ______________________________________                                    

As apparent from Table 13, when the content of Si is in the range of 10at % to 60 at %, high C/N values can be obtained. When the content of Siis 70 at % or higher, C/N is decreased because the exchange bondingforce between the reproducing layer 18' and the recording layer 22 issmall.

According to the present invention, a magneto-optical recording mediumwhich can effect high-density recording can be provided. Further, thepresent invention has an effect that a mark adjacent to a mark to bereproduced can be perfectly masked to improve a reproduction output. Inaddition, the crosstalk can also be improved.

What is claimed is:
 1. A magneto-optical recording medium comprising:afirst magnetic layer having an easy axis of magnetization in a plane ata room temperature; a second magnetic layer formed over said firstmagnetic layer and having an easy axis of magnetization perpendicular toa film plane; and a third magnetic layer formed over said secondmagnetic layer and having an easy axis of magnetization perpendicular tosaid film plane;wherein a Curie temperature Tc1 of said first magneticlayer, a Curie temperature Tc2 of said second magnetic layer, and aCurie temperature Tc3 of said third magnetic layer are related tosatisfy Tc1>Tc2 and Tc3>Tc2, and wherein when a reproducing laser beamis directed onto the medium, a beam spot is formed on the medium havinga temperature distribution including a low temperature area, anintermediate temperature area and high temperature area, said high andlow temperature areas functioning as masks to inhibit reading in thoseareas, while only said intermediate temperature area is available forreading.
 2. A magneto-optical recording medium according to claim 1,further comprising a nonmagnetic intermediate layer interposed betweensaid first magnetic layer and said second magnetic layer.
 3. Amagneto-optical recording medium according to claim 1, furthercomprising a nonmagnetic intermediate layer interposed between saidsecond magnetic layer and said third magnetic layer.
 4. Amagneto-optical recording medium according to claim 1, wherein each ofsaid first, second, and third magnetic layers is formed from a rareearth-transition metal amorphous alloy film.
 5. A magneto-opticalrecording medium comprising:a first magnetic layer having an easy axisof magnetization in a plane at a room temperature; a second magneticlayer formed over said first magnetic layer and having an easy axis ofmagnetization in a plane at said room temperature; and a third magneticlayer formed over said second magnetic layer and having an easy axis ofmagnetization perpendicular to a film plane;wherein a Curie temperatureTc1 of said first magnetic layer, a Curie temperature Tc2 of said secondmagnetic layer, and a Curie temperature Tc3 of said third magnetic layerare related to satisfy Tc1>Tc2 and Tc3>Tc2, and wherein when areproducing laser beam is directed onto the medium forming a beam spoton said medium having a temperature distribution including a lowtemperature area, an intermediate temperature area and a hightemperature area, said high and low temperature areas functioning asmasks to inhibit reading in those areas, while only said intermediatetemperature area is available for reading.
 6. A magneto-opticalrecording medium according to claim 5, further comprising a nonmagneticintermediate layer interposed between said first magnetic layer and saidsecond magnetic layer.
 7. A magneto-optical recording medium accordingto claim 5, further comprising a nonmagnetic intermediate layerinterposed between said second magnetic layer and said third magneticlayer.
 8. A magneto-optical recording medium according to claim 5,wherein each of said first, second, and third magnetic layers is formedfrom a rare earth-transition metal amorphous alloy film.
 9. Amagneto-optical recording medium according to claim 8, wherein amagnetic moment of rare earth metal of said first magnetic layer ispredominant over a magnetic moment of transition metal of said firstmagnetic layer at said room temperature.
 10. A magneto-optical recordingmedium according to claim 9, wherein said first magnetic layer containsat least Gd and Fe.
 11. A magneto-optical recording medium according toclaim 10, wherein said first magnetic layer contains 26 at % to 35 at %of Gd.
 12. A magneto-optical recording medium according to claim 8,wherein a magnetic moment of rare earth metal of said second magneticlayer is predominant over a magnetic moment of transition metal of saidsecond magnetic layer at said room temperature.
 13. A magneto-opticalrecording medium according to claim 12, wherein said second magneticlayer contains at least Gd and Fe.
 14. A magneto-optical recordingmedium according to claim 12, wherein said second magnetic layercontains at least Dy and Fe.
 15. A magneto-optical recording mediumaccording to claim 13, wherein said second magnetic layer contains 26 at% to 35 at % of Gd.
 16. A magneto-optical recording medium according toclaim 12, wherein said second magnetic layer contains a nonmagneticmaterial selected from the group consisting of Si, Al, and Ti.
 17. Amagneto-optical recording medium according to claim 16, wherein saidsecond magnetic layer contains said nonmagnetic material in an amount of60 at % or less.
 18. A magneto-optical recording medium comprising:afirst magnetic layer having an easy axis of magnetization perpendicularto a film plane; a second magnetic layer formed over said first magneticlayer and having an easy axis of magnetization in a plane at a roomtemperature; a third magnetic layer formed over said second magneticlayer and having an easy axis of magnetization perpendicular to saidfilm plane; and a fourth magnetic layer formed over said third magneticlayer and having an easy axis of magnetization perpendicular to saidfilm plane;wherein a Curie temperature Tc1 of said first magnetic layer,a Curie temperature Tc2 of said second magnetic layer, a Curietemperature Tc3 of said third magnetic layer, and a Curie temperatureTc4 of said fourth magnetic layer are related to satisfy Tc1>Tc3,Tc2>Tc3, and Tc4>Tc3; and a coercive force Hc1 of said first magneticlayer at said room temperature, a coercive force Hc2 of said secondmagnetic layer at said room temperature, a coercive force Hc3 of saidthird magnetic layer at said room temperature, and a coercive force Hc4of said fourth magnetic layer at said room temperature are related tosatisfy Hc4>Hc3 and Hc4>Hc1, and wherein when a reproducing laser beamis directed onto the medium, a beam spot is formed on the medium havinga temperature distribution including a low temperature area, anintermediate temperature area, and a high temperature area, said highand low temperature areas functioning as masks to inhibit reading inthose areas, while only said intermediate temperature area is availablefor reading.
 19. A magneto-optical recording medium according to claim18, wherein said third magnetic layer and said fourth magnetic layer aremagnetically bonded together so as to satisfy Hc3<9σw/(2Ms3·h3) whereMs3 represents a value of saturation magnetization of said thirdmagnetic layer, h3 represents a thickness of said third magnetic layer,and σw represents a domain wall energy between said third magnetic layerand said fourth magnetic layer.
 20. A magneto-optical recording mediumaccording to claim 18, wherein each of said first, second, third, andfourth magnetic layers is formed from a rare earth-transition metalalloy film.
 21. A magneto-optical recording medium comprising:a firstmagnetic layer having an easy axis of magnetization perpendicular to afilm plane; a second magnetic layer formed over said first magneticlayer and having an easy axis of magnetization in a plane at a roomtemperature; and a third magnetic layer formed over said second magneticlayer and having an easy axis of magnetization perpendicular to saidfilm plane;wherein a Curie temperature Tc1 of said first magnetic layer,a Curie temperature Tc2 of said second magnetic layer, and a Curietemperature Tc3 of said third magnetic layer are related to satisfyTc1>Tc2 and Tc3>Tc2; and a coercive force Hc1 of said first magneticlayer at said room temperature and a coercive force Hc3 of said thirdmagnetic layer at said room temperature are related to satisfy Hc3>Hc1,and wherein when a reproducing laser beam is directed onto the medium, abeam spot is formed on the medium having a temperature distributionincluding a low temperature area, an intermediate temperature area, anda high temperature area, said high and low temperature areas functioningas masks to inhibit reading in those areas, while only said intermediatetemperature area is available for reading.
 22. A magneto-opticalrecording medium according to claim 21, wherein each of said first,second, and third magnetic layers is formed from a rare earth-transitionmetal alloy film.
 23. A magneto-optical recording medium according toclaim 22, wherein each of said first and second magnetic layers isformed from an amorphous alloy film containing Gd, Fe, and Co.
 24. Amagneto-optical recording medium according to claim 22, wherein saidsecond magnetic layer is formed from an amorphous alloy film containingGd and Fe.
 25. A magneto-optical recording medium according to claim 22,wherein said second magnetic layer contains a nonmagnetic materialselected from the group consisting of Si, Al, and Ti.
 26. A method forreproducing information recorded on a magneto-optical recording mediumcomprising a first magnetic layer having an easy axis of magnetizationin a plane at a room temperature; a second magnetic layer formed oversaid first magnetic layer and having an easy axis of magneticperpendicular to a film plane; and a third magnetic layer formed oversaid second magnetic layer and having an easy axis of magnetizationperpendicular to said film plane; wherein a Curie temperature Tc1 ofsaid first magnetic layer, a Curie temperature Tc2 of said thirdmagnetic layer and a Curie temperature Tc3 of said third magnetic layerare related to satisfy Tc1>Tc2 and Tc3>Tc2; said reproducing methodcomprising the steps of:directing a laser beam onto said recordingmedium while applying a bias magnetic field to heat said recordingmedium to temperatures lower than the Curie temperature of said thirdmagnetic layer; and forming a temperature distribution in a beam spot,said temperature distribution comprising a first area where thedirection of magnetization of said first magnetic layer is in anin-plane direction, a second area where magnetization of said thirdmagnetic layer is transferred to said second magnetic layer and saidfirst magnetic layer, and a third area where the temperature of saidsecond magnetic layer becomes higher than its Curie temperature.
 27. Amethod for reproducing information recorded on a magneto-opticalrecording medium comprising a first magnetic layer having an easy axisof magnetization in a plane at a room temperature; a second magneticlayer formed over said first magnetic layer and having an easy axis ofmagnetization in a plane at said room temperature; and a third magneticlayer formed over said second magnetic layer and having an easy axis ofmagnetization perpendicular to a film plane; wherein a Curie temperatureTc1 of said first magnetic layer, a Curie temperature Tc2 of said secondmagnetic layer, and a Curie temperature Tc3 of said third magnetic layerare related to satisfy Tc1>Tc2 and Tc3>Tc2; said reproducing methodcomprising the steps of:directing a laser beam onto said recordingmedium while applying a bias magnetic field to heat said recordingmedium to temperatures lower than the Curie temperature of said thirdmagnetic layer; and forming a temperature distribution in a beam spot,said temperature distribution comprising a first area where thedirection of magnetization of said first magnetic layer is in anin-plane direction, a second area where magnetization of said thirdmagnetic layer is transferred to said second magnetic layer and saidfirst magnetic layer, and a third area where the temperature of saidsecond magnetic layer becomes higher than its Curie temperature.
 28. Amethod for reproducing information recorded on a magneto-opticalrecording medium comprising a first magnetic layer having an easy axisof magnetization in a plane at a room temperature; a second magneticlayer formed over said first magnetic layer and having an easy axis ofmagnetization in a plane at said room temperature; and a third magneticlayer formed over said second magnetic layer and having an easy axis ofmagnetization perpendicular to a film plane; wherein a Curie temperatureTc1 of said first magnetic layer, a Curie temperature Tc2 of said secondmagnetic layer, and a Curie temperature Tc3 of said third magnetic layerare related to satisfy Tc1>Tc2 and Tc3>Tc2; said reproducing methodcomprising the steps:directing a laser beam onto said recording mediumto heat said recording medium to temperatures lower than the Curietemperature of said third magnetic layer; and forming a temperaturedistribution in a beam spot, said temperature distribution comprising afirst area where the direction of magnetization of said first magneticlayer is in an in-plane direction, a second area where magnetization ofsaid third magnetic layer is transferred to said second magnetic layerand said first magnetic layer, and a third area where the temperature ofsaid second magnetic layer becomes higher than its Curie temperature.29. A method for reproducing information recorded on a magneto-opticalrecording medium comprising a first magnetic layer having an easy axisof magnetization perpendicular to a film plane; a second magnetic layerformed over said first magnetic layer and having an easy axis ofmagnetization in a plane at a room temperature; a third magnetic layerformed over said second magnetic layer and having an easy axis ofmagnetization perpendicular to said film plane; and a fourth magneticlayer formed over said third magnetic layer and having an easy axis ofmagnetization perpendicular to said film plane; wherein a Curietemperature Tc1 of said first magnetic layer, a Curie temperature Tc2 ofsaid second magnetic layer, a Curie temperature Tc3 of said thirdmagnetic layer, and a Curie temperature Tc4 of said fourth magneticlayer are related to satisfy Tc1>Tc3, Tc2>Tc3, and Tc4>Tc3; and acoercive force Hc1 of said first magnetic layer at said roomtemperature, a coercive force Hc2 of said second magnetic layer at saidroom temperature, a coercive force Hc3 of said third magnetic layer atsaid room temperature, and a coercive force Hc4 of said fourth magneticlayer at said room temperature are related to satisfy Hc4>Hc3 andHc4>Hc1; said reproducing method comprising the steps of:directing alaser beam onto said recording medium while applying a bias magneticfield to heat said recording medium to temperature lower than the Curietemperature of said fourth magnetic layer; and forming a temperaturedistribution in a beam spot, said temperature distribution comprising afirst area where the direction of magnetization of said second magneticlayer is in an in-plane direction, a second area where magnetization ofsaid fourth magnetic layer is transferred to said third magnetic layer,said second magnetic layer and said first area magnetic layer, and athird area where the temperature of said third magnetic layer becomeshigher than its Curie temperature.
 30. A method for reproducinginformation recorded on a magneto-optical recording medium comprising afirst magnetic layer having an easy axis of magnetization perpendicularto a film plane; a second magnetic layer formed over said first magneticlayer and having an easy axis of magnetization in a plane at a roomtemperature; and a third magnetic layer formed over said second magneticlayer and having an easy axis of magnetization perpendicular to saidfilm plane; wherein a Curie temperature Tc1 of said first magneticlayer, a Curie temperature Tc2 of said second magnetic layer are, and aCurie temperature Tc3 of said third magnetic layer are related tosatisfy Tc1>Tc2 and Tc3>Tc2; and a coercive force Hc1 of said firstmagnetic layer at said room temperature and a coercive force Hc3 of saidthird magnetic layer at said room temperature are related to satisfyHc3>Hc1; said reproducing method comprising the steps of:directing alaser beam onto said recording medium while applying a bias magneticfield to heat said recording medium to temperatures lower than the Curietemperature of said third magnetic layer; and forming a temperaturedistribution in a beam spot, said temperature distribution comprising afirst area where the direction of magnetization of said first magneticlayer is identical with the direction of said bias magnetic field, asecond area where magnetization of said third magnetic layer istransferred to said second magnetic layer and said first magnetic layer,and a third area where the temperature of said second magnetic layerbecomes higher than its Curie temperature and the direction ofmagnetization of said first magnetic layer is identical with thedirection of said bias magnetic field.
 31. A magneto-optical recordingmedium comprising:a first magnetic layer having a Curie temperature Tc1; a second magnetic layer formed over said first magnetic layer andhaving a Curie temperature Tc2; a third magnetic layer formed over saidsecond magnetic layer, said third magnetic layer having an easy axis ofmagnetization perpendicular to a film plane and a Curie temperature Tc3;andwherein at least one of said first and second magnetic layers has aneasy axis of magnetization in a plane at a room temperature, whereineither one of said Curie temperatures Tc1 and Tc2 is lower than saidCurie temperature Tc3, and wherein when a reproducing laser beam isdirected onto the medium, a beam spot is formed on said medium having atemperature distribution including a low temperature area, anintermediate temperature area and high temperature area, said high andlow temperature areas functioning as masks to inhibit reading in thoseareas, while only said intermediate temperature area is available forreading.
 32. A magneto-optical recording medium according to claim 31,wherein an in-plane mask is formed in said low temperature area having atemperature lower than the Curie temperature Tc1, and wherein a mask isformed in said high temperature area having a temperature higher thanthe Curie temperature Tc2 of said second magnetic layer.
 33. A methodfor reproducing information recorded on a magneto-optical recordingmedium comprising a first magnetic layer having a Curie temperature Tc1; a second magnetic layer formed over said first magnetic layer andhaving a Curie temperature Tc2; a third magnetic layer formed over saidsecond magnetic layer, said third magnetic layer having an easy axis ofmagnetization perpendicular to a film plane and a Curie temperature Tc3;wherein at least one of said first and second magnetic layers has aneasy axis of magnetization in a plane at a room temperature and whereineither one of said Curie temperatures Tc1 and Tc2 is lower than saidCurie temperature Tc3; said method comprising the steps of:directing alaser beam onto said recording medium while applying a bias magneticfield to heat said recording medium to temperatures lower than the Curietemperature Tc3 of said third magnetic layer; and forming a temperaturedistribution in a beam spot said temperature distribution comprising afirst area where the direction of magnetization of at least one of saidfirst and second magnetic layers is in an in-plane direction, a secondarea where magnetization of said third magnetic layer is transferred tosaid second magnetic layer and said first magnetic layer, and a thirdarea where a temperature of either one of said first and second magneticlayers becomes higher than the respective Curie temperatures Tc1 andTc2.
 34. A reproducing method according to claim 33 wherein a directionof said bias magnetic field is identical to a direction of magneticfield applied for recording information on said magneto-opticalrecording medium.